app: make sure the projection buffer exists in gimp_projection_pickable_flush()

This does not make any difference for normal images, but for images
without display (like when creating thumbnails, or for GUI-less
scripting), this is now needed because we create the initial
projection much more lazily, and got an empty projection.

app/core/gimpprojection.c

@@ -229,6 +229,9 @@ gimp_projection_pickable_flush (GimpPickable *pickable)
 {
   GimpProjection *proj = GIMP_PROJECTION (pickable);
 
+  /* create the buffer if it doesn't exist */
+  gimp_projection_get_buffer (pickable);
+
   gimp_projection_finish_draw (proj);
   gimp_projection_flush_now (proj);