Add function avfilter_graph_parse_ptr() and favor it in place of
avfilter_graph_parse(), which will be restored with the old/Libav
signature at the next bump.

If HAVE_INCOMPATIBLE_LIBAV_API is enabled it will use the
Libav-compatible signature for avfilter_graph_parse().

At the next major bump the current implementation of
avfilter_graph_parse() should be dropped in favor of the Libav/old
implementation.

Should address trac ticket #2672.