Expose multi-frame methods for SkCodecImageGenerator

This will allow Flutter to avoid some usages of SkCodec
and have consistent EXIF handling for multi-frame formats
that support EXIF.

Change-Id: I80daee9b00777d88d0a960cc0f0d76da6680e257
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/302270
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Dan Field <dnfield@google.com>
2 files changed