Auto merge of #1396 - glennw:opt-rects, r=kvark
Auto merge of #1396 - glennw:opt-rects, r=kvark

Optimize alpha rectangle primitives.

On the MotionMark Multiply test, this saves:
 * 2ms in backend.
 * 6ms in compositor.

It drops the draw calls in half for that particular test.

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