Switch to a single render pass for the whole frame, as opposed to
a render pass per draw call. This should be a large improvement to
performance. This currently requires some unsafe to work around a
self-reference between RenderPass and CommandEncoder in Frame;
this could eventually be cleaned up by changing RenderBackend
to return a Frame object instead of using begin_frame/end_frame
pairs.
Also switch to using push constants for the transform/color
uniforms.
Mike Welsh