Clean up SkScalerContext descriptor creation

Allow a descriptor creation using just a rec and effects.
This is the most complex part of the change.

Cleanup SkScalerContextFlags to be a real thing.
This has spread everywhere.

BUG=skia:7515

Change-Id: I67189c9aeb51f1777a1c60145216524d440cb890
Reviewed-on: https://skia-review.googlesource.com/98320
Commit-Queue: Herb Derby <herb@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
13 files changed