RGB levels on MIXER level for color grading

[Sidonai-1]

Description

This has become a very requested feature when working with videowalls.
Right now we use ffmpeg filters to adjust the RGB levels on videos that need small corrections, when it's just one video it's perfect, but the filters come with some drawbacks:

• Video needs to be played again 'from the beginning' for the filter to take effect
• No color grading on image producer
• When there are 30 videos with the same filter needs you have to apply it to all, instead of doing it just once on the CasparCG layer
• ffmpeg filters take lots of resources, if you add them up together some combinations result in drops on the ffmpeg producer

Of course we always try to avoid taking all this work on us and request a correct version of the videos, but for the times this is not doable, this feature would be a life saver.

Edit: Related to #82

[5opr4ni]

I agree, a much wanted feature.