spice: flush on GL update before notifying client

spice: flush on GL update before notifying client

Since the introduction of spice/virgl support in commit
474114b7 ("spice: add opengl/virgl/dmabuf support"), the drawing isn't
being flushed before notifying the client. This results in
outdated/sluggish drawing on client side, in particular when using the
Linux console.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210216092056.2301293-1-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>