media-video/mpv: require only virtual/opengl for raspberry-pi in 9999

media-video/mpv: require only virtual/opengl for raspberry-pi in 9999

Both libEGL and libGLESv2 from mesa are not only unnecessary, but can
even cause issues if picked instead of libs from raspberrypi-userland.
At runtime we have environment set up correctly by raspberrypi-userland.
At build time correct libs are enforced by the next commit.

We still need GL/gl.h header provided by virtual/opengl though.

Package-Manager: Portage-2.3.3, Repoman-2.3.1
Closes: https://github.com/gentoo/gentoo/pull/3718