Add lossless mode for encoders

[stderr-to-devnull]

The current encoders implementation is extremely limited, making it impossible to even get close to transparency, at any settings.

The output contains very bad blocking and banding even with following settings:

• hardware h.264 / h.265, quality 1
• x264, preset veryslow, quality 0, tune disabled or film
• x265, preset slower, quality 0, tune disabled

Just implement the lossless switch from x264: -qp 0, that's it, nothing else required.