Fix cases of variable shadowing in rasterization.

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

Change-Id: I169ff3bac8517869132297c8e98bd9bd4248a349
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436100
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Herb Derby <herb@google.com>
5 files changed