Runtime effects: Fix error when mutating main's coords

These often became direct references to a varying, which can't be
mutated in GLSL. Fix this by copying them to a local variable in the
FP, and pointing all references at that.

Bug: skia:10918
Change-Id: I705e3c966b1d44fc4dfc3d4b40eb8e46110febdd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334043
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed