ext/gdk_pixbuf/gstgdkpixbuf.c: Some cleanups.

ext/gdk_pixbuf/gstgdkpixbuf.c: Some cleanups.

Original commit message from CVS:
* ext/gdk_pixbuf/gstgdkpixbuf.c: (gst_gdk_pixbuf_sink_setcaps),
(gst_gdk_pixbuf_get_capslist), (gst_gdk_pixbuf_sink_getcaps),
(gst_gdk_pixbuf_class_init), (gst_gdk_pixbuf_init),
(gst_gdk_pixbuf_flush), (gst_gdk_pixbuf_sink_event),
(gst_gdk_pixbuf_chain):
Some cleanups.
Added RGBA as a possible output format.
Correctly free the supported mimetypes.
deprecate silent arg, it's not used.
Return result from _alloc_buffer to peer.