Sets the pixel format to be used by subsequent calls to set_gfx_mode()
and create_bitmap(). Valid depths are 8 (the default), 15, 16, 24, and 32
bits. Example:
set_color_depth(32);if(set_gfx_mode(GFX_AUTODETECT, 640, 480, 0, 0)!=0){
abort_on_error("Couldn't set a 32 bit color resolution");}
Note that the screen color depth won't change until the next successful
call to set_gfx_mode().