Generalize composing imagefilters and shaders to blenders

The preexisting enum versions now are just shallow factories for
the new blender versions, though internally we've kept the
specializations on impl.

Change-Id: I3449682bb443a4ff9f53cc7b0860343c56e377e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/424436
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
8 files changed