x11: if the WM supports _NET_FRAME_EXTENTS, don't wait for map

Some window managers can prevent mapping of a window as a feature. i3
can put new windows on a certain workspace (with "assign"), so if mpv is
started on a different workspace, the window will never be mapped.

mpv currently waits until the window is mapped (blocking almost all of
the player), in order to avoid race conditions regarding the window
size. We don't want to remove this, but on the other hand we also don't
want to block the player forever in these situations.

So what we need is a way to know when the window manager is "done" with
processing the map request. Unfortunately, there doesn't seem to be a
standard way for this. So, instead we could do some arbitrary
communication with the WM, that may act as "barrier" after map request
and the "immediate" mapping of the window. If the window is not mapped
after this barrier, it means the window manager decided to delay the
mapping indefinitely. Use the _NET_REQUEST_FRAME_EXTENTS message as such
a barrier. WMs supporting this message must set the _NET_FRAME_EXTENTS
property on the mpv window, and we receive a PropertyNotify event. If
that happens, we always continue and cancel waiting for the MapNotify
event.

I don't know if this is sane or if there's a better mechanism. Also,
this works only for WMs which support this message, which are not many.
But at least it appears to work on i3. It may reintroduce flickering on
fullscreen with other WMs, though.

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