goom: Ensure src caps are writable

In some cases the src caps determined by goom weren't writable, causing
a bunch of assertion failures and failed caps. Fixed by always
explicitely making the caps writable

https://bugzilla.gnome.org/show_bug.cgi?id=705475

gst/goom/gstgoom.c

@@ -279,6 +279,7 @@ gst_goom_src_negotiate (GstGoom * goom)
     target = templ;
   }
 
+  target = gst_caps_make_writable (target);
   structure = gst_caps_get_structure (target, 0);
   gst_structure_fixate_field_nearest_int (structure, "width", DEFAULT_WIDTH);
   gst_structure_fixate_field_nearest_int (structure, "height", DEFAULT_HEIGHT);