RGB output hooks

[tzarc]

Description

The Djinn has the ability to dynamically change the current limit of the RGB LEDs on the PCB.
When they pull too much current, they brownout and stop working, unsurprisingly at higher brightness values.

This PR introduces a hook that can be overridden by keyboards in order to dynamically modify the output RGB values with code, for example scaling the max brightness depending on the USBPD negotiation state:

RGB rgblight_hsv_to_rgb(HSV hsv) {
    float scale = 0.2f;
    if (current_3000mA) {
        scale = 0.55f;
    } else if (current_1500mA) {
        scale = 0.32f
    }

    hsv.v = (uint8_t)(hsv.v * scale);
    return hsv_to_rgb(hsv);
}

Unsure if we need documentation for this -- it's pretty low-level and unlikely to be used terribly frequently.

Types of Changes

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

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]

Shiny!

Also does that mean you're working on power negotiation? :D

[tzarc]

Shiny!

Also does that mean you're working on power negotiation? :D

......maybe. :P