Fixes incorrect PipelineCompiler::compile_pipeline() step_mode (#2126)

There's what might be considered a proper bug in `PipelineCompiler::compile_pipeline()`, where it overwrites the `step_mode` for the passed in `VertexBufferLayout` with `InputStepMode::Vertex`. Due to this some ugly workarounds are needed to do any kind of instancing.

In the somewhat longer term, `PipelineCompiler::compile_pipeline()` should probably also handle a `Vec<VertexBufferLayout>`, but that would be a (slightly) larger PR, rather than a bugfix. And I'd love to have this fix in sooner than we can deal with a bigger PR.

crates/bevy_render/src/pipeline/pipeline_compiler.rs

@@ -205,7 +205,7 @@ impl PipelineCompiler {
 
         // the vertex buffer descriptor that will be used for this pipeline
         let mut compiled_vertex_buffer_descriptor = VertexBufferLayout {
-            step_mode: InputStepMode::Vertex,
+            step_mode: mesh_vertex_buffer_layout.step_mode,
             stride: mesh_vertex_buffer_layout.stride,
             ..Default::default()
         };