add guard for handling swizzles via rasterpipeline

Bug: skia:
Change-Id: I26499659c15bf79a840246a2eb6eb064e43b4c32
Reviewed-on: https://skia-review.googlesource.com/21163
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed