Further ffmpeg optimizations

From https://www.reddit.com/r/linux/comments/aa1jzm/bashcaster_an_actually_simple_screen_recorder_for/ecozlfj/

> Suggestions for gif recording:
>
> use the mpdecimate filter to remove duplicate frames. In a screen recording there can be huge number of duplicate frames, especially if it's mostly showing a still image of a desktop. Video games can have frame drops, which manifest as duplicate frames in recodings. This should reduce the file size of the resulting gif significantly.
>
> use bayer dithering, which will yield another signficant size reduction.
>
> In a 10 second desktop test recording that I converted to a gif, these two methods lead to a file size reduction of 89% (!) when compared to a gif that just used a plain pallettegen and palletteuse filter complex with no other additions.
>
> I usually use something like this to create gifs:
>
> ffmpeg -i input.mp4 -vsync 0 -filter_complex "[0:v] scale=-1:360,mpdecimate,fps=15,split [a][b];[a] palettegen=max_colors=64 [p];[b][p] paletteuse=dither=bayer:bayer_scale=5" output.gif