vo_xv: use new option API
options: move --colorkey option to vo_xv
vo_opengl_old: use new option API
vo_direct3d: use new option API
vo_opengl: some option changes

Doing "mpv --vo=opengl:lscale=help" now lists possible scalers and
exits. The "backend" suboption behaves similar. Make the "stereo"
suboption a choice, instead of using magic integer values.