[svg] Add gradientUnits attribute, value, and parsing

Specifying gradientUnits will allow gradient coordinates to be specified
relative to object bounding boxes, as seen in test 'coords-units-01-b'.

Not yet used with this CL.

Bug: skia:10842
Change-Id: I6038cf3995a94c7e3a7ac73ad8305872353a403c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328977
Reviewed-by: Florin Malita <fmalita@chromium.org>
Auto-Submit: Tyler Denniston <tdenniston@google.com>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
8 files changed