Blender Git Loki

Fix failing assert & uninitialized paint settings in empty scene

Steps to reproduce were:
* Add new (empty) scene
* Add some mesh object
* Change to texture paint mode

While it can be argued that we should already init the settings when
creating a new scene (so that the texture tab can show up),
BKE_paint_init() should always ensure paint settings are initialized
either way.

Related to T73611.