Make blur surface with bounds of layer

This CL changes the blur logic to take a snapshot only of the layer, not
the whole screen.

This has 2 benefits:
1. The blur shader samples only within the bounds of the surface, so
this change fixes a black frame around the blurred layer
2. The blur surface is smaller when the layer is not fullscreen, so less
information gets copied around

Bug: 174464919
Test: check background blur doesn't have a black frame
Test: check blur regions are in place
Change-Id: Ic31742248cc8441225503af48f13e6ef95a59443