Add experimental API to draw a set of SkImages in one SkCanvas call.

The client provides a src and dst rect for each image as well as
a bitfield that indicates whether each edge of the image should be
antialiased. This per-edge AA is useful for tiled compositors.

Rather than take a full SkPaint this API only takes an alpha, a filter
quality (which is pinned to kLow), and a blend mode. This is a likely
point of future evolution.

Currently the API is only fully implemented for kSrcOver on the GPU
backend. With other blend modes or on other backends AA will be ignored
for images that do not have all four edge AA flags set.

BUG: skia:8444

Change-Id: I143998dda8ad6a25f64e18cd600392ba553030ac
Reviewed-on: https://skia-review.googlesource.com/c/159062
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
43 files changed