Improve multi-window render clipping logic

Fixes: 28125010

Restructures 'scene defer', to implement window backdrop overdraw
avoidance in new render pipeline, and disable clipping to content draw
bounds.

Also restructures FrameBuilder's constructors, to separate out into
multiple defer methods.

Change-Id: I53facb904c1a4a4acc493d8a489921a79a50494e