Added SkSL workaround for devices which cannot safely access gl_FragCoord

This is the root cause of https://github.com/flutter/flutter/issues/13216
I've got a GM that demonstrates the bug, but only in Viewer.

Bug: skia:7410
Change-Id: Iaa1f27b10166aa09e4dc5949e5a6ca1bd14c99ac
Reviewed-on: https://skia-review.googlesource.com/93920
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed