SF: Allow display layer clip bounds to be overriden

Add a new virtual call to get the display bounds (in layer space) when
used as the root clip rectangle for all layers, so that it can be
customized where needed.

The base functionality is not changed -- the display viewport is still
used as the clip rectangle.

Test: Builds
Bug: 154780821
Change-Id: I5a8c564cacd87e37d11128ab4742d9f2ee897bf2