Load some GL funtions if USE_SHADERS is defined.

git-svn-id: https://www.crowproductions.de/repos/prboom/branches/prboom-plus-24/prboom2@4279 df9a1a3e-e9e3-0310-a482-b5a01ba5a74e
Added "shaders" sector light mode.

git-svn-id: https://www.crowproductions.de/repos/prboom/branches/prboom-plus-24/prboom2@4280 df9a1a3e-e9e3-0310-a482-b5a01ba5a74e