Improve Stretch shader performance

Refactor stretch shader to return within
conditional blocks instead of falling
through to main function body.
Improves performance on coral by approximately 60%
~600 fps to ~1k fps on non-surfaceview case
~300 fps to 500 fps on surfaceview case

Bug: 187718492
Test: manual
Change-Id: Ida0d0dee9c94b0ac210a024708100283fecc2a5c