Fix cases of variable shadowing in SkSL.

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

Change-Id: Ic582c59b9f7dfee2d7e90e50bfb36c57e958c673
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436641
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
10 files changed