Add options for better quality using the Theora/Vorbis encoder

[berarma]

Currently, Shutter Encoder creates only CBR files that don't provide the best quality at a given bitrate.

Adding options for using the quality settings (-q:v) with range [0,10] would enable VBR encoding and allow the best quality with this codec. Same for the Vorbis audio encoder, option -q:a and range [-1, 10].

The Theora encoder allows the use of the GOP option like other video codecs but it isn't available in the interface. FFmpeg defaults to GOP=12 that falls too short for most content. A better default is GOP=64.

I'm proposing to reference Shutter Encoder in the Godot docs for use by its community since Theora is the only built-in video encoder in the game engine, but with the current defaults and options, the results are the worst possible.

Thanks for the tool. It's a great GUI for FFmpeg.