OpenGL palette rendering and related things

src/hardware/hw_cache.c

src/hardware/hw_defs.h

src/hardware/hw_drv.h

src/hardware/hw_glob.h

src/hardware/hw_main.c

src/hardware/hw_main.h

src/hardware/hw_shaders.c

src/hardware/r_opengl/r_opengl.c

src/m_menu.c

src/r_data.c

src/r_defs.h

src/sdl/hwsym_sdl.c

src/sdl/i_video.c

src/st_stuff.c