Fixes #16705 : digital rain follows val

[KoenGoe]

RGB matrix digital rain animation did not follow the current set hsv value.
Due to this, it also ignored RGB_MATRIX_MAXIMUM_BRIGHTNESS, which might cause too much current to be drawn .

This pull request changes the algorithm to adapt to the currently set hsv value by both

• scaling the brightness constants with the value
• adjusting the speed of decay to keep time from full brightness to 0 roughly constant

• Core
• Bugfix
• Enhancement/optimization

Issues Fixed or Closed by This PR

Fixes 16705

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]

Not sure it should be added here, but I have noticed that this animation doesn't respect the led flags setting, and will render on leds that aren't "enabled".

[KoenGoe]

It also doesn't do RGB_MATRIX_USE_LIMITS(led_min, led_max);
I'm not very familiar with how this works and what it means in terms of how often the animation function is called.
If the function gets called more often (/second) according to these limits, it would unintentionally change the animation speed with the current implementation