We basically use a couple primitives to draw these
(PaintRoundedRectWithRadius, FillRect), so making the code a bit generic
implementing stuff with WebRender seems straight-forward.
I've kept using the fallback codepath for the bits that draw complex
paths like arrows and such, but the rest of the things should work with
this patch.
A thing I'm not too happy about is the scrollbar painting setup (requires a lot
of boilerplate), but modulo template hacks make nsNativeBasicTheme a template
that receives its super class as a parameter or something) it seems hard to do
better.
Differential Revision: https://phabricator.services.mozilla.com/D105931
cb286403d0