Blender Git Loki

Engine specific panel's
- All of this is in python and easy to change.
- each panel class has a set() of compatible engines.
- this set is checked for the poll function
- external engines can add themselves to this panels compatible engines
eg.
buttons_world.WORLD_PT_mist.COMPAT_ENGINES.add('POVRAY_RENDER')

I tried doing this by subclassing each panel and replacing only the poll function to reference 'POVRAY_RENDER' but it became fairly complicated and meant registering many of the same panels under different names.

Added mist support to povray.