Stretch breaks on wembley

Work around GPU driver issues on Wembley.
Refactor the overscroll stretch shader to
explicitly return values in the helper methodsd
instead of using out parameters to store the
result.

Fixes: 182566543
Test: manual
Change-Id: Ib9c57d7cb3de06bc7155fa2af364d70cbf2ea812
1 file changed