Rework r_shadow_shadowmode to be more maintainable and understandable - it now caches...

Rework r_shadow_shadowmode to be more maintainable and understandable - it now caches the values of r_shadow_shadowmapping and r_shadow_deferred, and is now updated when those change, rather than just at startup, and changing those now also causes a recompile of world lights (which always had different bounds when shadowing was on/off, toggling shadows had weird behavior as a result, so this is a bugfix).

This fixes a massive fps drain with settings that disabled shadows entirely by disabling shadowmapping (where r_shadow_shadowmapping 0 and r_shadow_deferred 0 together caused it to reallocate the shadowmap texture every frame and do other bad stuff)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12562 d7cf8633-e32d-0410-b094-e92efae38249