wgpu requires buffer copy sizes and offsets to be 4-byte aligned.
Unfortunately, ActionScript can perform 2-byte aligned uploads
into an IndexBuffer3D.
To support this, we now keep a copy of the IndexBuffer3D on the CPU.
When performing an upload to the buffer, we round the offset down
and the size up to the nearest 4-byte aligned value. The cpu buffer
is used to fill out the write with existing data, so that we don't
corrupt the contents of the GPU buffer.
To avoid introducing a new RefCell, I've changed IndexBuffer3D
to use a `Box` instead of an `Rc` to store the trait object.
This allows us to pass a mutable reference down to the backend.
e488cc9f7a