Support DynamicProperties for gradient start/end points?

[Gankra]

In https://bugzilla.mozilla.org/show_bug.cgi?id=1465792 it appears that webrender is getting pegged by a progress bar with an animated linear-gradient, where the animation is just the offset of the gradient being moved.

Not sure how high priority this is, it sounds like non-trivial gecko work may also be required.

[kvark]

Should be fairly trivial on WR side, and nice to have anyhow.