media-video/mpv: use more sane logic for opengl and egl USE flags

media-video/mpv: use more sane logic for opengl and egl USE flags

Currently 'opengl' USE stands for 'anything that draws using OpenGL
API', and 'egl' USE stands for 'EGL for X11'. Notice these are not
complementary.

Thus, for example, on Wayland one should use 'mpv[wayland,opengl,-egl]',
which is a bit counterintuitive, since Wayland (compositor) uses EGL
only. Also one cannot disable GLX support, but leave EGL on X11, because
of USE flags interdependencies.

Both X11 and Wayland use OpenGL API to do drawing, but historically the
first one was native OpenGL in X11 aka GLX and later came EGL in X11 and
Wayland. However, now there is a tendency to prefer EGL to not bind
oneself to X11.

There is no global 'egl' USE atm, but the logic of local 'egl' USEs
across the tree is 'not bind oneself to GLX'. On Linux there is no need
to repeat that drawing is done using OpenGL API even when EGL comes to
play.

Taking all of the above into account more sane and expected meaning
should be given to 'opengl' and 'egl' USE flags.

After this change 'opengl' USE stands for 'OpenGL in X11 aka GLX' and
'egl' USE stands for 'any EGL-based output'. Notice these are now
complementary.

Package-Manager: portage-2.2.24