Add hex interpolator

[dariusj]

Add hex, hexa, rgb, and rgba custom interpolators.

[davesmith00000]

(This work relates to issue #94.)

Good to see you are the hack event yesterday!

So to get this over the line, I think we need the following:

• Ensure the code works for vec3's and vec4's, i.e. hex"#FF00FFFF" ends up as vec4(1.0f, 0.0f, 1.0f, 1.0f).
• Standard issue simple unit tests for both to prove it.
• An (can be just one) acceptance test with both of them in it, that produces the expected GLSL. There's a pattern that's easy enough to follow I think. I'd expect syntax like vec4(hex"#FF00FF", 1.0f) to produce vec4(vec3(1.0,0.0,1.0),1.0). If it doesn't, we'll need to work out why not. Fair warning, if things are going to unexpectedly go sideways, this is where it'll happen, so don't struggle with it for too long if it's not happening for you - I'll help! 😄

I'm very happy if the work ends there, but the follow on piece is the same thing but for something like: rgb"255,0,0", and rgba"255,0,0,255".

[davesmith00000]

Great work! 💪

I've tried to answer your comments, give me a shout if you need help!

[davesmith00000]

When you get back to this @dariusj, can you please sync your fork and rebase. I've fiddled with the build a bit. Thanks!

[davesmith00000]

Looks great! Assuming the build passes I'm happy to get this merged. I have left a couple of comments on the error messaging, but they aren't show stoppers.

[davesmith00000]

If you're happy, you could take this out of draft?