sRGB display transform encoding in Metal backend seems to be using the wrong formula

[exavi]

For some reason the exponent is set to 2.2 instead of 2.4 making all outputs of MaterialXView be a bit bogus if used as a "source of truth".

I created a PR with the two line fix that solves it but looks like there's a bunch of things I'd need to do/sign to even get that considered and I'm a bit busy right now :(

This is the PR, I believe it should be enough to get it to match with externally set up real-time renderers that happen to use the MaterialXView for verification

#2846

[exavi]

closing now that the fix is merged, thanks!