vo_gpu: x11egl: support Mesa OML sync extension

Mesa supports the EGL_CHROMIUM_sync_control extension, and it's
available out of the box with AMD drivers. In practice, this is exactly
the same as GLX_OML_sync_control, but for EGL. The extension
specification is separate from the GLX one though, and buried somewhere
in the Chromium code.

This appears to work, although I don't know if it really works.

In theory, this could be useful for other EGL targets. Support code for
it could have been added to egl_helpers.c to avoid some minor duplicated
glue code if another EGL target were to provide this extension. I didn't
bother with that. ANGLE on Windows can't support it, because the
extension spec. explicitly requires POSIX timers. ANGLE on Linux/OSX is
actively harmful for mpv and hopefully won't ever use it. Wayland uses
EGL, but has its own fancy presentation feedback stuff (and besides, I
don't think basic video player functionality works on Wayland at all).
context_drm_egl maybe? But I think DRM has its own stuff.

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