Support rendering meshes with per-vertex color

[thomasp85]

Thanks for all the work on this. I'm currently using it as a basis for a data viz focused rendering pipeline.

Allowing mesh rendering would greatly expand the power of Vello for e.g. rendering paths with variable width/stroke color etc. I'm currently battling with getting rid of AA seams when rendering the triangles individually.

[nicoburns]

I believe this would also enable using Vello as the graphics backend for GUI toolkits like Iced and egui.

[thomasp85]

Yes. A general mesh render brings a lot of flexibility