Make texture painting work

[Schwarzbaer]

3D View's texture paint mode does not work when Panda 3D is set as renderer; The texture is considered missing, even when it is present. This occurs when creating a Diffuse texture in Panda 3D renderer mode, or when switching to it (in which case the texture "disappears" from the model). The texture itself exists / is created, and can be seen in the UV Image editor. When switching back to Blender Renderer, the texture appears again on the model automatically.