Only throw_if in setOption if setting an option failed, fix filtergraph parsing, add sample aspect ratio accessors

[rkfg]

We shouldn't throw if av_opt_set returned zero (success). Also fixed segfault in filtergraph parsing. We only need to manually free AVFilterInOut structures after using avfilter_graph_parse_ptr as in the example but we use avfilter_graph_parse which frees automatically (and if we do the same after the program segfaults). Hence all the unique_ptr machinery is not needed. Although the filter classes certainly needs some work to make them easier to use and more abstract, like, the in/out filters could be created automatically and we'd only provide the actual filters.

Another feature for the filters is easy access to sample (pixel) aspect ratio in video frame and codec context. It's used in filter input initialization, and while unnecessary would be good to have it exposed for consistency.

[h4tr3d]

Applied, thanks!