A built-in vertex painter

[PrecisionRender]

I think Godot should have the ability to paint vertex colors onto 3D models implemented directly into the engine. We do have plugins like VPainter that implement this functionality, but they're not without bugs and limitations. (VPainter doesn't officially support 4.0 yet.) Vertex painting is a really useful technique to achieve a higher-quality look in 3D, even with low-poly/retro games. I think it should either be a built-in tool, or more likely, an officially supported addon like Terrain will be.

[Calinou]

This requires #6486 to be implemented first to be done in a non-destructive way.