[graphite] Handle opaque Layer fast-path in recordDraw() instead of searchBackwards()

This will let some of the different search/pilot paths consolidate in
follow-up refactors.

Bug: 419535595
Change-Id: I362091a1d7c5cb6e588ea45507706df7681404a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1283278
Reviewed-by: Thomas Smith <thomsmit@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>