Switch SkImage::CompressionType to be an enum class

Following CLs use CompressionType much more widely so let's switch it first.

Bug: skia:9680
Change-Id: Ie8847f24936e90dd85e0ad79cbe5542d68c87b12
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/258882
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
12 files changed