This commit adds a new keycode `RGB_SMOD` which is the same as `RGB_MOD` (cycle through all modes),
but when it is used in combination with shift it will reverse the direction.
* Address #1689 by using a formula to define the breathing curve and exposing defines to control the shape of the curve.
* Tweak the behavior of breathing for clueboard
* Allow the knight animation to be restricted to a portion of the LED strip
* Add keys for jumping directly to particular animation modes
* Remove orphaned break statements
* Tweak the `RGB_MODE` buttons so they cycle through the same mode.
* small indentation fix
Colin T.A. Gray