- since the clip planes for plane mirrors did not work anymore I reimplemented them using shader based logic. It still needs to be seen if this affects performance on older hardware.
- some shader fixes.
- for some reason using world coordinates for clipping in the shader is somewhat imprecise so the clip plane heights have to be adjusted a bit for it.
- added some checks to the wall rendering code that will allow to disable the clip planes in many cases, even when a plane mirror portal is active. This also solves the precision issue with using world coordinates for clip checks.