GUACAMOLE-2167: Fix for incorrect handling of the ALTGR key.

guacamole-common-js/src/main/webapp/modules/Keyboard.js

@@ -1519,8 +1519,8 @@ Guacamole.Keyboard.ModifierState.fromKeyboardEvent = function(e) {
 
     // Assign states from old flags
     state.shift = e.shiftKey;
-    state.ctrl  = e.ctrlKey;
-    state.alt   = e.altKey;
+    state.ctrl  = e.getModifierState('AltGraph') || e.ctrlKey;
+    state.alt   = e.getModifierState('AltGraph') || e.altKey;
     state.meta  = e.metaKey;
 
     // Use DOM3 getModifierState() for others