THE DRAGON HAS BEEN SLAIN

Numlock is fully supported, as well as shift lock. Of course, I
cannot properly test shift lock or caps lock because I have neither
of those things, but I assume they work as well as num lock does.

backends/x/x.go

@@ -43,6 +43,13 @@ type Backend struct {
 		descent      int
 	}
 
+	modifierMasks struct {
+		capsLock   uint16
+		shiftLock  uint16
+		numLock    uint16
+		modeSwitch uint16
+	}
+
 	windowBoundsClean bool
 }