Replace SkSpecialImage::makeTightSubset with asImage

Replace SkSpecialImage::makeTightSubset with asImage

This should allow the relanding of:

https://skia-review.googlesource.com/c/8450/ (Remove asTextureRef from SkSpecialImage & update effects accordingly (take 2))

Change-Id: I517af11036e3f44a280bbe6b9275f6c38f4a86a4
Reviewed-on: https://skia-review.googlesource.com/8498
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>