Clip to perspective quad in viewer

For GMs that draw content outside the window, this ensures that fake
and real perspective look more similar.

Change-Id: I5fd2372a21a0b6c04cb33ddef4ff53aebda1c43d
Reviewed-on: https://skia-review.googlesource.com/129454
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed