waylandsink: do not render twice the same buffer

waylandsink: do not render twice the same buffer

Do not try to render a buffer that is already being rendered.
This happens typically during the initial rendering stage as the first
buffer is rendered twice: first by preroll(), then by render().
This commit avoids this assertion failure:
  CRITICAL: gst_wayland_compositor_acquire_buffer: assertion
  'meta->used_by_compositor == FALSE' failed

https://bugzilla.gnome.org/show_bug.cgi?id=738069

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>