apply the projection's viewport to the visibleregion passed to hwc

each desplay's projection's viewport essentially clips each layer,
so this should be reflected in the visibleregion passed to
h/w composer.

DisplayDevice getViewport and getFrame are now guaranteed to return
valid Rects.

Change-Id: I4c25f34fb26af10179eb26d429ca6c384c671e91
3 files changed