Another mergeLayers optimization

Since we pre-fill the cached `keyMap` with the value of `DefaultLayer`, there is
no need to check that layer again, looking for a non-transparent key. Whatever
is there, will be used anyway.

This way we save a cycle for keys that are transparent everywhere but the
default layer.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

src/layers.cpp

@@ -64,7 +64,7 @@ Layer_::mergeLayers(void) {
         for (uint8_t c = 0; c < COLS; c++) {
             int8_t layer = highestLayer;
 
-            while (layer >= DefaultLayer) {
+            while (layer > DefaultLayer) {
                 if (Layer.isOn (layer)) {
                     Key mappedKey;