vo_gl: refuse to use software renderer unless explicitly requested

vo_gl: refuse to use software renderer unless explicitly requested

vo_gl will now fail at initialization if a software renderer is
detected. This is the same behavior as vo_gl_nosw. Making this the
default behavior is preferable, because it will simplify positioning
vo_gl in the VO autoprobe list (video_out_drivers[]). Also, vo_gl_nosw
exists only if X11 support is configured.

Move gl in place of gl_nosw. Add the "sw" suboption to vo_gl to allow
using vo_gl even if a software renderer is detected.

vo_gl_nosw is now completely equivalent to vo_gl. It is kept to avoid
breaking too many user configurations, but should be considered
deprecated.