[svg] Several filter helper tweaks

- Allow resolving inputs without modifying their colorspace
- Add helper to get the colorspace of a particular input
- Make resolveColorspace() virtual and add filter context to its
  signature

Bug: skia:10841
Change-Id: I2e226ec26205f527c2d171140072f106ec35fbe0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/356416
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
11 files changed