FFMPEG Core Feature Update

[kaysedwards]

Description

I added some functionality and changed the controls to expose the new functionality. The biggest change is that save states work for most (I say most because one can't seek through some broken by design test files I have available.) files. I've also added a seek that increments or decrements the position in the file according to a strength which cycles through a fixed set of seconds by again pressing the relevant button. I also changed the audio and subtitle controls to allow more handling by moving forwards or backwards through the available streams within the given file. Finally, I made a null subtitle state so that subtitles can be disabled by simply cycling through the available streams.

[sonninnos]

New warning

cores/libretro-ffmpeg/ffmpeg_core.c: In function 'seek_adjust':
cores/libretro-ffmpeg/ffmpeg_core.c:630:1: warning: control reaches end of non-void function [-Wreturn-type]
  630 | }
      | ^

[kaysedwards]

New warning

cores/libretro-ffmpeg/ffmpeg_core.c: In function 'seek_adjust':
cores/libretro-ffmpeg/ffmpeg_core.c:630:1: warning: control reaches end of non-void function [-Wreturn-type]
  630 | }
      | ^

Oh, I'm sorry!

I'll add a default return to seek_adjust in order to silence the warning (It should return zero by default to account for quirks, but I can't think of any current compiler that wouldn't zero the global.) and fix the support message in retro_load_game to properly report that the standard seek is thirty seconds.

If you'd like I can do those adjustments right away, but I'm thinking of playing around with the core a bit more to try and fix seeing (I don't mean actually seek to the correct position, but I think I can at least prevent the core from crashing RetroArch when trying to seek bad files.) of broken files, add a configuration option for seek strength, add a configuration option for default audio, add a configuration option for default subtitles, and allow the controls to be adjusted a bit.