Correctly report the transparent region

First, we fix the transparent region computation to use the order as the
drawing. Previously, it is using the tree traverse order, not the draw order.

Second, add the y offset for any view with positive z value, this will allow
some space for shadow.

b/30124573

Change-Id: I98d38261ffd346b762651e087cb243e45fed6952
2 files changed