LED Matrix: Implement CIE1931 curve

[fauxpark]

Description

Brightness changes are much nicer on the eyes now. Also tidying up a few other things in preparation for adding animations and updating the suspend- and split-related code.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[drashna]

To be honest, I really, really don't like the LED_DISABLE_WHEN_USB_SUSPENDED (or it's rgb sibbling), in how it's implemented.

Maybe have the function ifdef-ed, instead, and just have a single call.
eg:

void led_matrix_set_suspend_state(bool state) {
#ifdef LED_DISABLE_WHEN_USB_SUSPENDED
    if (state) {
        led_matrix_set_value_all(0);  // turn off all LEDs when suspending
    }
    g_suspend_state = state;
#endif
}

And shove the call into the core suspend/wake functions. This way, no code is needed in the keyboard or user levels. And you only need the single define (and not set to true or false).

[fauxpark]

Yeah, that's a good idea, but I think I'll hold off on that until LED Matrix is in sync with RGB Matrix, then they can both be improved as one.

[drashna]

Not a problem. It's been something that has been on my mind for a while now.