Properly draw the window background on window resize

Bug #7385090

This change gets rid of two silly asumptions:
- That a layer needs to be cleared with opaque black (it shouldn't,
it's already cleared to transparent and the view will cover it up
with its own background)
- The the clip should be dirty at the beginning of a frame only
when the render target is opaque

Change-Id: I415b6d3cab196057fb0281419a53fef601a44e28