moved a lot of 2D/screen stuff out of gl_screen.c into cl_screen.c

moved a lot of 2D/screen stuff out of gl_screen.c into cl_screen.c
moved remainder of gl_screen.c into gl_backend.c
removed gl_screen.c
inlined GL_BrightenScreen in gl_draw.c
moved a lot of GL stuff out of gl_rmain.c into gl_backend.c
converted polyblend to R_Mesh based
replaced lighthalf/gl_lightmode with v_overbrightbits
renamed contrast to brightness, and brightness to contrast (to match monitor controls)
merged hardware and software brightness/contrast/gamma into v_contrast, v_brightness, v_gamma, etc, switchable using v_hwgamma (automatically forced off if hardware gamma support is unavailable)
added ghostable menu items (unavailable gamma, for example)
rearranged options menu
2D resolution slider added (from 320x240 to full resolution)
fixed misuse of strncmp for bindings menu (thanks Elric)
adjustable crosshair size
renamed GL_BeginRendering to VID_GetWindowSize
renamed GL_EndRendering to VID_Finish
made screenshot and envmap share one function for grabbing images
moved underwater warping into cl_screen code
starting work on new entity protocol (not hooked up yet)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@1523 d7cf8633-e32d-0410-b094-e92efae38249