Reset transparent region when saving a surface

Fixes: 28432088

Ensure a transparent region on a saved surface is reset for future use,
since the surface should be like-new if used again.

This prevents an issue where the region - used to signal a portion of
content doesn't need to be composited - is persisted when a saved
surface is reused. The client assumes it's new and in default
state (composite everything), but the window is clipped when composited.

Change-Id: Icf2ec94c735679d715aded58de7eab12e9c43367