Remove fixed function state osg::ClipPlane

Remove osg::ClipPlane entirely from OpenMW.

This is part of larger effort to remove fixed function state and allow for rendering openmw with a core profile.

https://registry.khronos.org/OpenGL/extensions/EXT/EXT_clip_cull_distance.txt extension exists, which would allow us to emulate usage exactly, but support for this is reported to be extremely low so is not practical.

A stopgap solution could be to use the discard-in-fragment-shader-approach and switch over to native clip planes once core profile is supported.