vo_opengl: minor simplification

The parameter to default_tex_params() is always the same.
vo_opengl: better probe handling

Involve detection of software renderers in the probing properly. Other
VOs could handle probing also more gracefully, and e.g. produce less
noise if an API is unavailable. (Although other than the OpenGL VOs,
only vo_wayland will.)

Now the "sw" suboption for vo_opengl[_old] is strictly speaking not
needed anymore. Doing "--vo=opengl" disables the probing logic, and will
always force it, if possible.

Includes some simplifications as well.
vo_opengl: fail if required functions are missing

Before this, missing additional but required functions were ignored.
("Main" functions still made it error out.) But we pretty much expect
that these are present on a given version level, and only an extremely
buggy OpenGL implementation would not do this.
vo_opengl, x11: add ES context creation via GLX

Apparently GLX can do this, so using EGL is not strictly required.

This code tries to create an ES context if creating a desktop GL context
fails. Also, a "x11es" backend for forcing ES (instead of desktop GL) is
added, mostly for testing and debugging.
vo_opengl: skip GLES when autoprobing

There are probably bugs with GLES support; also, if you somehow get GLES
instead of desktop GL on a desktop computer, something else is probably
wrong. So I see no point in using this automatically. We first need to
find out whether the GLES support works on real hardware, and whether
it is useful at all.