Expose public factories for platform decoders

Bug: skia:8076

Move SkImageGeneratorCG and SkImageGeneratorWIC to include/ports/,
and make them SK_API.

Remove SkImageGeneratorCG::NewFromEncodedCG. No known clients are using
it, and it was a private API in src/. There is already a
MakeFromEncodedCG to replace it.

Similarly, switch WIC from New to Make.

Add a compile test for using them with
SetImageGeneratorFromEncodedDataFactory.

Change-Id: I897eb7a887b3736a6c614a68e38f38b6a1942cf1
Reviewed-on: https://skia-review.googlesource.com/137387
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
6 files changed