Rescaled grey16be colors using ffmpegs colorlevels to directly correct the original issue

[itay-grudev]

This PR re-scales the grey16be colors using ffmpeg's colorlevels filter to directly reverse the original issue causing the image to be really dark.

Importantly, using this method the quality of small details is preserved, as opposed to gamma, brightness and contrast filters which significantly reduce the quality of text and small details and/or do not achieve the desired brightening effect.

Current solution on main since #100:

Improved version with colorlevels:

Fixes: #96

[itay-grudev]

@rien The quality of the text is preserved with this fix and the brightness is at the levels it was before the v3 software update.

[rien]

Ah yes, that looks way better indeed. Thank you for the PR!