ext/libmng/: Added basic MNG decoder. Needs more work. The encoder does not work...

ext/libmng/: Added basic MNG decoder. Needs more work. The encoder does not work yet.

Original commit message from CVS:
* ext/libmng/Makefile.am:
* ext/libmng/gstmng.c: (plugin_init):
* ext/libmng/gstmng.h:
* ext/libmng/gstmngdec.c: (gst_mngdec_get_type),
(gst_mngdec_base_init), (gst_mngdec_class_init),
(gst_mngdec_sinklink), (gst_mngdec_init), (gst_mngdec_src_getcaps),
(gst_mngdec_loop), (gst_mngdec_get_property),
(gst_mngdec_set_property), (mngdec_error), (mngdec_openstream),
(mngdec_closestream), (mngdec_handle_sink_event),
(mngdec_readdata), (mngdec_gettickcount), (mngdec_settimer),
(mngdec_processheader), (mngdec_getcanvasline), (mngdec_refresh),
(gst_mngdec_change_state):
* ext/libmng/gstmngdec.h:
* ext/libmng/gstmngenc.c: (gst_mngenc_get_type),
(mng_caps_factory), (raw_caps_factory), (gst_mngenc_base_init),
(gst_mngenc_class_init), (gst_mngenc_sinklink), (gst_mngenc_init),
(gst_mngenc_chain), (gst_mngenc_get_property),
(gst_mngenc_set_property):
* ext/libmng/gstmngenc.h:
Added basic MNG decoder. Needs more work. The encoder does
not work yet.