Enable setting style for text-based subtitles

[Petr Nejedly]

The subtitles filter now has force_style attribute. Shold be able to set font / color / size.

[Petr Nejedly]

requires new ffmpeg

[Petr Nejedly]

looks like we can use this for color and size:

:force_style="PrimaryColour=&H00FF00\,Fontsize=20\,FontName=Mistral"

color is &H<BGR>

Default values are: Arial, 16, &HFFFFFF

[Petr Nejedly]

HTML color picker:

<input type='color' name='color2' value='#3355cc' />