ext/gdk_pixbuf/: Add gdkpixbufsink element for easy snapshotting (#525946).

ext/gdk_pixbuf/: Add gdkpixbufsink element for easy snapshotting (#525946).

Original commit message from CVS:
* ext/gdk_pixbuf/Makefile.am:
* ext/gdk_pixbuf/gstgdkpixbuf.c: (plugin_init):
* ext/gdk_pixbuf/gstgdkpixbufsink.c:
(gst_gdk_pixbuf_sink_base_init),
(gst_gdk_pixbuf_sink_class_init), (gst_gdk_pixbuf_sink_init),
(gst_gdk_pixbuf_sink_start), (gst_gdk_pixbuf_sink_stop),
(gst_gdk_pixbuf_sink_set_caps),
(gst_gdk_pixbuf_sink_pixbuf_destroy_notify),
(gst_gdk_pixbuf_sink_get_pixbuf_from_buffer),
(gst_gdk_pixbuf_sink_handle_buffer), (gst_gdk_pixbuf_sink_preroll),
(gst_gdk_pixbuf_sink_render), (gst_gdk_pixbuf_sink_set_property),
(gst_gdk_pixbuf_sink_get_property):
* ext/gdk_pixbuf/gstgdkpixbufsink.h:
Add gdkpixbufsink element for easy snapshotting (#525946).