[sksl] Support FB fetch in Dawn

Bug: b/238756863
Change-Id: I3da7293c50c7e202f0eed71ba931a5abbb721ff8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/779456
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
8 files changed