[svg] Add support for `var` in color parsing.

In OT-SVG `var` support is required for palette use. Even without
palette support this is needed to get the fallback color.

Change-Id: Iea8c4502b57933115f5cbf44fcc183fef19da984
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/534140
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
9 files changed