Improve Stretch shader performance more

Refactor stretch shader to always work in
skia's coordinate system instead of normalizing
the input coordinates.
Improves performance on coral by approximately 10-20% across
various use cases on top of the branching refactoring
perf improvements
~1k fps to ~1.1k fps on non-surfaceview cases
~500 fps to ~600 fps o surfaceview cases

Bug: 187718492
Test: manual
Change-Id: I0b36f48699739979948bfec160a8aa64fc80b7a7