st-private: Receive a CoglFramebuffer

st-private: Receive a CoglFramebuffer

Same case of the previous patch; _st_paint_shadow_with_opacity()
uses cogl_get_draw_framebuffer(), and this patch makes it receive
a CoglFramebuffer as a parameter instead.

The cautious reader might notice that this commit apparently goes
against the long-term goal, for it introduces more instances of
cogl_get_draw_framebuffer(). This is not wrong, but these introduced
ones will be removed later on, when ClutterActor.paint() receives
a CoglFramebuffer as a parameter instead.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/283