Add SkBlender class; thread into SkVMBlitter.

Like SkColorFilter, SkShader, etc., this has a public-facing component
(SkBlender) and a private subclass (SkBlenderBase) which can be
obtained via a helper function (as_BB). At present there are no public-
facing methods, but the type needs to be exposed to be usable by the
outside world.

These classes exist for SkRuntimeEffect to subclass. The blender base
provides a `program` method with the parameters that blending will use.

Change-Id: I75c772fd4108a9c21fbda84201a8b23d3750a0df
Bug: skia:12080
Auto-Submit: John Stiles <>
Commit-Queue: Brian Osman <>
Reviewed-by: Brian Osman <>
14 files changed