Add a workaround for inaccurate interpolants on Adreno 3xx.

Also ensure that sk_FragCoord x and y values are at pixel centers when
workaround is used.

Change-Id: Ib748af9e496a406a50622e00e96e1346cbb5eb26
Reviewed-on: https://skia-review.googlesource.com/97064
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
7 files changed