This repo contains a setup to render directly in the texture of a mesh. The code renders the mesh using commandbuffer in the uv space of the mesh's texture. For more have a look at the blog post: https://medium.com/@shahriyarshahrabi/mesh-texture-painting-in-unity-using-shaders-8eb7fc31221c
Edit: Added support for painting in Albedo, Metalic and Gloss.
There is no support for mipmapping at the moment.