media-video/mpv: map opengl USE to 'opengl' video output in 9999

media-video/mpv: map opengl USE to 'opengl' video output in 9999

Pros:
- mapping is clear for everyone w/o getting into technical details;
- wayland users are very happy by default.

Cons:
- you cannot build X11 backend with EGL, but without GLX.

Cons are very minor and ebuild complexity stays at about the same level,
but wayland users should be much happier with the default configuration.

Previous opengl USE behaviour is described in Gentoo commit 411d5bb4746.
As one can see it was somewhat unnecessarily complex.

Gentoo-Bug: 608858
Package-Manager: Portage-2.3.5, Repoman-2.3.2