reorganized view rendering setup code a bit to reduce potential for state bugs (more...

reorganized view rendering setup code a bit to reduce potential for state bugs (more thorough and consistent state resets now)
added support for glStencilOpSeparate function in OpenGL 2.0 (or GL_ATI_separate_stencil extension prior to that), so ATI cards should get a speed boost now, this path is now also used on NVIDIA hardware, but support for GL_EXT_stencil_two_side extension has been retained to support several other vendors

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6714 d7cf8633-e32d-0410-b094-e92efae38249