Fix matrix scoping issue in sample

Change-Id: Id0023a12a21957ec9236d4ffaa03134995388d60
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278456
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed