Use Layer.highest() instead of Layer.top()

We are fine with the cached value, so lets use that and save a little time,
rather than the always-correct `Layer.top()`.

Signed-off-by: Gergely Nagy <kaleidoscope@gergo.csillger.hu>

src/Kaleidoscope/Colormap.cpp

@@ -43,12 +43,12 @@ void ColormapEffect::max_layers(uint8_t max_) {
 }
 
 void ColormapEffect::onActivate(void) {
-  last_highest_layer_ = Layer.top();
+  last_highest_layer_ = Layer.highest();
   ::LEDPaletteTheme.updateHandler(map_base_, last_highest_layer_);
 }
 
 void ColormapEffect::update(void) {
-  if (Layer.top() == last_highest_layer_)
+  if (Layer.highest() == last_highest_layer_)
     return;
 
   onActivate();