This PR implements core 'stage3D' APIs. We are now able
to render at least two demos from the Context3D docs - a simple
triangle render, and a rotating cube.
Implemented in this PR:
* Stage3D access and Context3D creation
* IndexBuffer3D and VertexBuffer3D creation, uploading, and usage
* Program3D uploading and usage (via `naga-agal`)
* Context3D: configureBackBuffer, clear, drawTriangles, and present
Not yet implemented:
* Any 'dispose()' methods
* Depth and stencil buffers
* Context3D texture apis
* Scissor rectangle
General implementation strategy:
A new `Object` variant is added for each of the Stage3D objects
(VertexBuffer3D, Program3D, etc). This stores a handle to the
parent `Context3D`, and (depending on the object) a handle
to the underlying native resource, via `Rc<dyn
SomeRenderBackendTrait>`).
Calling methods on Context3D does not usually result in an immediate
call to a `wgpu` method. Instead, we queue up commands in our
`Context3D` instance, and execute them all on a call to `present`.
This avoids some nasty wgpu lifetime issues, and is very similar
to the approah we use for normal rendering.
The actual rendering happens on a `Texture`, with dimensions
determined by `createBackBuffer`. During 'Stage' rendering,
we render all of these Stage3D textures *behind* the normal
stage (but in front of the overall stage background color).
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