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This can leave the window displaying out-of-date contents, which might never be updated thereafter.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.
Fixes: #1171 (closed)