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This version of the API takes an alpha per quad rather than one
alpha for the set. This will allow the image sets to span multiple
shared_quad_states (this is future work).

Bug: skia:8563
Change-Id: Idc2c46da9e6274414bfc510408c14c79aa5c37a2
Reviewed-on: https://chromium-review.googlesource.com/c/1345091Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Cr-Commit-Position: refs/heads/master@{#610897}