Split SAA scan converter into its own file

AAA was already in SkScan_AAAPath. SAA was implemented along with some
of the top-level dispatch code in SkScan_AntiPath. Now, AntiPath just
has the dispatch logic, and SAAPath holds the actual SAA scan converter.

Also moved a static function only used in one spot out of SkScanPriv.h
and into the cpp.

Change-Id: I7055940f512fe2d54893a72586d07d02ce348a1c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/657998
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed