Change avfilter_open() signature, from:

Change avfilter_open() signature, from:
AVFilterContext *avfilter_open(AVFilter *filter, const char *inst_name);
to:
int avfilter_open(AVFilterContext **filter_ctx, AVFilter *filter, const char *inst_name);

This way it is possible to propagate an error code telling the reason
of the failure.

Originally committed as revision 24765 to svn://svn.ffmpeg.org/ffmpeg/trunk