[FEATURE REQUEST] Scale film grain synthesis strength with average frame luma value

[trixoniisama]

Sometimes you might want to make film grain synthesis more or less noticeable in darker scenes compared to brighter ones.

Therefore, I believe introducing a new parameter (naming proposition: fgs-luma-scaling) that would, like explicitly written, scale the synthesis strength depending on the frame average luma value, could enable more tweaking potential to a tool that clearly lacks customizability currently.