This is based on D15160 with some changes. Not all functionality is implemented yet.
Mainly note that this is just building a field from the node tree and evaluates that whenever the texture is evaluated. For batch evaluation, this approach works perfectly fine currently. For single element evaluation, I have to refactor things a bit.

The file below contains three different use cases:

• Painting on image directly.
• Using texture in displacement modifier.
• Painting texture in 3D view.

Issue is, none of these code paths seem to pass geometry specific information to texture evaluation currently (e.g. vertex indices, or triangle index + barycentric weights). So not sure how to test that yet.