This adds new beginLayer and endLayer functions to open and close layers
in the canvas. While layers are active, draw calls operate on a separate
texture that gets composited to the parent output bitmap when the layer
is closed. An optional filter can be specified in beginLayer, allowing
effects to be applied to the layer's texture when it's composited its
parent.

Tests:
 https://github.com/web-platform-tests/wpt/tree/master/html/canvas/element/layers
 https://github.com/web-platform-tests/wpt/tree/master/html/canvas/offscreen/layers

Fixes whatwg#8476

This update addresses 3 main things:
- Support for CSS filter strings was added to the beginLayer API
- Unclosed layers are now never rasterized when the canvas is presented
  to the user. Instead, the content of the layer is preserved and will
  be rasterized in a later frame, if/when the layer is closed.
- Replied to the first round of review comments.

This will be moved to a separate pull request.

With this change, the context filters now applies to the layer's output
bitmap and is resetted to "none" when entering layers.

This removes the filter argument of the beginLayer API.
BeginLayerOptions can possibliy be added to the specification as a
follow-up.

The HTMLCanvasElement's bitmap was linkified to clarify to which bitmap
each uses of the word "bitmap" refers to (either the element's bitmap,
the context's top level output bitmap or the context's current output
bitmap).

This means that when entering a layer, the current transform now is
reset to the identity matrix. setTransform and getTransform are now
local to the current layer, meaning that calling setTransform sets a
matrix relative to the parent layer.