Changed the way HDRP volumetric clouds presets are handeled to allow …

…for blending.

This PR changes the way presets are described for three reasons:

- Allow the user to blending between various presets.
- Start from a preset and alter it slightly.
- Improving the usage of the samples to increase the quality at equivalent cost.

This PR also changes the actual presets and alters the aplitude distortion.
https://user-images.githubusercontent.com/42026998/154358834-bb0d46d5-3a5e-43e4-ac70-44a3d62c61fd.mp4
https://user-images.githubusercontent.com/42026998/154358882-5ed4e19d-a449-4fcf-9920-a30a73328143.mp4

Packages/com.unity.render-pipelines.high-definition/Documentation~/Override-Volumetric-Clouds.md

@@ -78,9 +78,9 @@ When importing these two map Textures, disable **sRGB**. For best results, do no
 | --------------------------------- | ------------------------------------------------------------ |
 | **Cloud Control**                 | Specifies the mode to control volumetric cloud properties. The options are:<br/>&#8226; **Simple**: Uses sliders and input fields to customize volumetric cloud shape properties. This mode generates a cloud map from the various properties and uses HDRP's internal cloud lookup table.<br/>&#8226; **Advanced**: Uses separate Textures to specify each cloud type and their coverage. This mode generates a cloud map from the various Textures and uses HDRP's internal cloud lookup table.<br/>&#8226; **Manual**: Uses the cloud map and lookup table you supply to render the clouds. For more information on the cloud map and cloud lookup table, see [Cloud map and cloud lookup table](#cloud-map-and-cloud-lookup-table). |
 | - **Cloud Preset**                | Specifies the preset to apply to the **Simple** mode properties. The options are: <br/>&#8226; **Sparse**: Smaller clouds that are spread apart.<br/>&#8226; **Cloudy**: Medium-sized clouds that partially cover the sky.<br/>&#8226; **Overcast**: A light layer of cloud that covers the entire sky. Some areas are less dense and let more light through, whereas other areas are more dense and appear darker.<br/>&#8226; **Storm Clouds**: Large dark clouds that cover most of the sky.<br/>&#8226; **Custom**: Exposes properties that control the shape of the clouds. <br/><br/>This property only appears if you set **Cloud Control** to **Simple**. |
-| - **Custom Density Curve**        |  Controls the density (Y axis) of the volumetric clouds as a function of the height (X Axis) inside the cloud volume. <br/><br/>This property only appears if you set **Cloud Control** to **Simple** and then set **Cloud Preset** to **Custom**.|
-| - **Custom Erosion Curve**        |  Controls the erosion (Y axis) of the volumetric clouds as a function of the height (X Axis) inside the cloud volume. <br/><br/>This property only appears if you set **Cloud Control** to **Simple** and then set **Cloud Preset** to **Custom**.|
-| - **Custom Ambient Occlusion Curve**        |  Controls the ambient occlusion (Y axis) of the volumetric clouds as a function of the height (X Axis) inside the cloud volume. <br/><br/>This property only appears if you set **Cloud Control** to **Simple** and then set **Cloud Preset** to **Custom**.|
+| - **Density Curve**        |  Controls the density (Y axis) of the volumetric clouds as a function of the height (X Axis) inside the cloud volume. <br/><br/>This property only appears if you set **Cloud Control** to **Simple**.|
+| - **Erosion Curve**        |  Controls the erosion (Y axis) of the volumetric clouds as a function of the height (X Axis) inside the cloud volume. <br/><br/>This property only appears if you set **Cloud Control** to **Simple**.|
+| - **Ambient Occlusion Curve**        |  Controls the ambient occlusion (Y axis) of the volumetric clouds as a function of the height (X Axis) inside the cloud volume. <br/><br/>This property only appears if you set **Cloud Control** to **Simple**.|
 | - **Cumulus Map**                 | Specifies a Texture that defines the distribution of clouds in the lower layer. <br/><br/>This property only appears if you set **Cloud Control** to **Advanced**. |
 | - **Cumulus Map Multiplier**      | The multiplier for the clouds specified in the **Cumulus Map**. A value of **0** completely hides the cumulus clouds. <br/><br/>This property only appears if you set **Cloud Control** to **Advanced**. |
 | - **Alto Stratus Map**            | Specifies a Texture that defines the distribution of clouds in the higher layer. <br/><br/>This property only appears if you set **Cloud Control** to **Advanced**. |
@@ -102,8 +102,8 @@ When importing these two map Textures, disable **sRGB**. For best results, do no
 | - **Erosion Factor**              | Controls the amount of erosion to apply on the edge of the clouds. A higher value erodes clouds more significantly. <br/><br/>This property only appears if you set **Cloud Control** to **Advanced** or **Custom**, or if you set it to **Simple** and then set **Cloud Preset** to **Custom**. |
 | - **Erosion Scale**               | Controls the size of the noise HDRP uses in the erosion stage to add local details to the cloud edges. <br/><br/>This property only appears if you set **Cloud Control** to **Advanced** or **Custom**, or if you set it to **Simple** and then set **Cloud Preset** to **Custom**. |
 | **Earth Curvature**               | The curvature of the cloud volume. This defines the distance at which the clouds intersect with the horizon. |
-| **Lowest Cloud Altitude**         | The altitude of the lowest cloud in meters.                  |
-| **Cloud Thickness**               | The thickness of the volumetric clouds volume in meters.     |
+| **Bottom Altitude**               | Controls the altitude of the bottom of the volumetric clouds volume in meters.|
+| **Altitude Range**                | Controls the size of the volumetric clouds volume in meters. |
 | **Fade In Mode**                  | Controls the mode in which the clouds fade in when close to the camera's near plane.|
 | **Fade In Start**                 | Controls the minimal distance at which clouds start appearing.|
 | **Fade In Distance**              | Controls the distance that it takes for the clouds to reach their complete density.|