[svg] Add support for funcIRI fallback colors

Per spec [1], funcIRI paint servers can specify a fallback color to be
used when the IRI is invalid.

[1] https://www.w3.org/TR/SVG11/painting.html#SpecifyingPaint

Bug: skia:11802
Change-Id: I9711822f97ed949315d944aff4de2fda97eaa607
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/389796
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
3 files changed