Clean up GLX function exports.

Clean up GLX function exports.

Make functions for the following extensions only accessable via
glXGetProcAddress:

    - GLX_EXT_texture_from_pixmap
    - GLX_MESA_copy_sub_buffer
    - GLX_MESA_release_buffers
    - GLX_MESA_swap_control
    - GLX_MESA_swap_frame_usage
    - GLX_OML_sync_control
    - GLX_SGI_make_current_read
    - GLX_SGI_swap_control
    - GLX_SGI_video_sync
    - GLX_SGIX_swap_barrier
    - GLX_SGIX_swap_group

Removed all remnants of the following extensions:

    - GLX_MESA_set_3dfx_mode
    - GLX_SGI_cushion
    - GLX_SGIX_dmbuffer
    - GLX_SGIX_video_resize
    - GLX_SGIX_video_source
    - GLX_SUN_get_transparent_index
    -