The new `.setBrightness()` and `.getBrightness()` methods control the brightness
of the LEDs, by dispatching them to the LED drivers. We dispatch to the drivers
so that nothing else needs to be aware of brightness control. Plugins will
always set the unadjusted colors, and anything and anyone who reads colors, will
also get the unadjusted values.
Pushing the adjustment down to the driver level makes everything smooth, and
since we do gamma correction there anyway, it makes sense to do brightness
adjustment at the same place, too.
Fixes#775.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>