xwayland/eglstream: flush stream after eglSwapBuffers

When eglSwapBuffers inserts a new frame into a window's stream, there may be a
delay before the state of the consumer end of the stream is updated to reflect
this. If the subsequent wl_surface_attach, wl_surface_damage, wl_surface_commit
calls are received by the compositor before then, it will (typically) re-use
the previous frame acquired from the stream instead of the latest one.

This can leave the window displaying out-of-date contents, which might never be
updated thereafter.

To fix this, after calling eglSwapBuffers, xwl_glamor_eglstream_post_damage
should call eglStreamFlushNV. This call will block until it can be guaranteed
that the state of the consumer end of the stream has been updated to reflect
that a new frame is available.

Closes: xorg/xserver#1171

Signed-off-by: Erik Kurzinger <ekurzinger@nvidia.com>
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