Blender Git Loki

Bugfix 18671 revisted

Node editor didn't support editing non-material texture node trees.
Campbell pointed me to fact it's been used already, like for brush
painting. However, this only worked via linking the texture to a
material... hackish stuff.

Now the Node Editor supports all other Textures too, with three extra
icon buttons to define which.

- Active Object: for textures linked to Materials or Lamps
- World: textures from Scene world.
- Brush: textures from active Brush

The latter can only be set and used when in Paint or Sculpt mode:

- Paint mode: in Image window, Paint Tool panel, set active brush
- Sculpt mode: in EditButtons, Texture panel, select empty slot, add texture.

Note that refreshes of previews in Node Editor is not always happening on
switching contextes. Just click a socket to refresh view.