Fix cases of variable shadowing in /src/gpu.

If we manage to fix all the existing cases of variable shadowing, we
could enable -Wshadow.

Change-Id: I3ad3c603f97e398382202d11c159d25cca0a7b9f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/438317
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
7 files changed