Clear offscreen surface in custommesh_cs to white

Makes the GM draw better in 1010102 configs

Bug: skia:12720
Change-Id: I587c542e602b6e2d972444a45aa222a646f8ff70
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/488176
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed