Gray fix2

[Mezage]

Still trying to understand how fragments work, advice would be nice.
Attempt at patching issue #1284

[Sailsman63]

Before settling on an implementation, it might be nice to define what kind of results we want from a greyscale transformation, which would be more useful in a shader. See https://en.wikipedia.org/wiki/HSL_and_HSV#Disadvantages for a quick comparison of some of the various ways of transforming RGB into "Lightness/Brightness" of some kind.

Even if the decision is to stick with the basic Arithmetic Mean, as implemented here (technically Intensity from the HSI color space) I question whether the gating check is worth it:

• You're adding two comparisons and a branch instruction per pixel, with a fairly high likelihood of having to do the sum and division anyway. I'd expect it to mostly be slower than just doing the math. (Though, someone would have to benchmark on a variety of hardware to be sure.)
• While the speed difference is likely to be minimal either way, you're adding cognitive overhead to the code.

[scenemax3d]

Hi @Mezage ,
Are you still interested in this PR?
If so, can you provide the following information:

1. Does it break any current feature / interface
2. How well it’s tested from 0 - 10
3. Does it needs any new documentation
4. Release notes

Thank you