If using an old sketch, ignore LayerCount

If it's an old sketch, LayerCount will default to 0, so in order for
Layer.on() to function, don't bother checking for out-of-bounds if
LayerCount == 0.

src/layers.cpp

@@ -129,9 +129,8 @@ void Layer_::move(uint8_t layer) {
 
 void Layer_::on(uint8_t layer) {
   // If we're trying to turn on a layer that doesn't exist; abort
-  if (layer >= LayerCount) {
+  if (LayerCount != 0 && layer >= LayerCount)
     return;
-  }
 
   bool wasOn = isOn(layer);