the visible region sent to HWC was garbage

we used to have a visibleRegion object per layer, but now
it's per screen; so at somepoint the code got changed to
calculate the per-screen visible region on the stack and that's
what got passed to HWC.

we're now setting the visibleRegionScreen at each frame and
freeing at after the HWC set() call. We use the underlaying
SharedBuffer so that in most cases we don't have to allocate,
free or copy memory around.

Bug: 7089478
Change-Id: I24fa556c76613a225d9fe7a6c6b727bb476144d8