take the "transparent region" into account for blending

until now it was only used to discard a layer entirely.
we're now reducing the size of the layer if it is still
visible, if possible.

this works for instance when a surfaceView is used and
only the menu bar is displayed over it.

Change-Id: I3f5527c5cd1e69ecc968272c8948f1513ada8c55
3 files changed