itsscale should accept formula expressions

[milahu]

currently, ffmpeg does not accept expressions for the itsscale parameter

$ ffmpeg -itsscale '25/(24000/1001)' -i src.mkv dst.mkv
Expected number for itsscale but found: 25/(24000/1001)

workaround: pass a float number to itsscale

$ echo 'scale=20; 25/(24000/1001)' | bc
1.04270833333333333333

$ ffmpeg -itsscale $(echo 'scale=20; 25/(24000/1001)' | bc) -i src.mkv dst.mkv

passing expressions already works for other parameters

$ ffmpeg -i src.mkv -af 'atempo=25/(24000/1001)' dst.mkv

would be nice to make this more consistent
so we can avoid extra calculations and rounding errors