Split out the left and right alt keys, to make it easier to emulate altgr.

This may or may not actually be useful.

key_defs.h

@@ -15,11 +15,12 @@ typedef union {
 
 #define KEY_FLAGS         B00000000
 #define CTRL_HELD         B00000001
-#define ALT_HELD          B00000010
-#define SHIFT_HELD        B00000100
-#define GUI_HELD          B00001000
-#define SWITCH_TO_KEYMAP  B00010000
-#define MOMENTARY         B00100000
+#define LALT_HELD         B00000010
+#define RALT_HELD         B00000010
+#define SHIFT_HELD        B00001000
+#define GUI_HELD          B00010000
+#define SWITCH_TO_KEYMAP  B00100000
+#define MOMENTARY         B01000000
 #define SYNTHETIC_KEY     B10000000
 
 // we assert that synthetic keys can never have keys held, so we reuse the _HELD bits

key_events.cpp

@@ -93,7 +93,10 @@ void press_key(Key mappedKey) {
     if (mappedKey.flags & CTRL_HELD) {
         Keyboard.press(Key_LCtrl.rawKey);
     }
-    if (mappedKey.flags & ALT_HELD) {
+    if (mappedKey.flags & LALT_HELD) {
+        Keyboard.press(Key_LAlt.rawKey);
+    }
+    if (mappedKey.flags & RALT_HELD) {
         Keyboard.press(Key_RAlt.rawKey);
     }
     if (mappedKey.flags & GUI_HELD) {