Auto merge of #1735 - glennw:ft2, r=mrobinson

Collapse LayerRectCalculationAndCullingPass into FrameBuilder.

Although this still looks quite messy, it should allow us to start
splitting up this functionality in a more fine-grained way in the
future. Specifically, the next step is to split off the way that
the clip stack is handled, so that this can be done once during
clip-scroll tree update - for performance reasons.

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Author: bors-servo