[svg] Refactor SkSVGImage to expose image loading

This is in preparation for reusing the image loading + aspect ratio
mapping for <feImage>.

Change-Id: I53dcf904476aae386f672488379201524855a703
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/404217
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
2 files changed