Define texture level mode
Set texture bias in constant mode
Merge pull request #5905 from raven02/patch-9

Define texture level mode and set bias in constant mode
Temporarily (?) revert ffdba45b4bbd because of massive perf drop in GT, working around #5900

Not sure what to do about the Chinese Tactics Ogre though...
Remove cached light variables to reduce work when parsing DLs.

Also removes some duplication between GL and the inactive D3D code.
Buildfix non-windows platforms