Fix GIF duration and add other format support

[KnowZero]

• Fixed gif duration via -count_frames ( Fixes issue #8 and #10 )
• Now uses codec_name for gif detection instead of filename
• Added support for webm(vp8, vp9), ogv(Theora) and mkv via -show_entries format
• When all options fail, use ffmpeg decode instead of ffprobe to gather data for formats like mpg (mpeg 2)
• Checks codec_type for video before setting which stream to use ( Should fix issue #5 )

[KnowZero]

@scottpetrovic - Hey scott, just wanted to know if I should continue adding features/fixing bugs (maybe try to fix the finding ffmpeg outside of envs and etc?) or is there other plans for this plugin?

[scottpetrovic]

@KnowZero - Ohh. I can take a look. I haven't look at this in a little while.

One thing that put this on the backburner was there was work on putting ffmpeg directly in Krita...which would simplify this a lot and it would remove a lot of this code. At least for Windows people, the whole environment variable thing is just too complicated for most people.

I could try to look at this patch. I must have forgotten about it.

[KnowZero]

@scottpetrovic - Hmm, so if ffmpeg is gonna be bundled with Krita, would it make sense for me to attempt to rewrite this in C++ and add it as a native feature?

And it's okay, we all get busy and distracted here and there.

Edit: Just to confirm, ffmpeg is just gonna be put as an executable? Or is the plan to access the shared libraries directly in c++?

[bdsm]

I was running into issue #5, tested to see if this worked as a fix and can confirm that this solves it - thank you @KnowZero, I appreciate your efforts.