implemented l2 and r2 mostly (except for some to-do items mentioned in comments)

ffi/glfw-input.c

@@ -70,6 +70,14 @@ float getLeftJoystickYAxisState() {
   }
 }
 
+float getL2State() {
+  return axes[2];
+}
+
+float getR2State() {
+  return axes[5];
+}
+
 int isCrossButtonPressed() {
   return state.buttons[GLFW_GAMEPAD_BUTTON_CROSS];
 }

ffi/glfw-input.sml

@@ -74,4 +74,9 @@ struct
     _import "isR1ButtonPressed" public : unit -> int;
   val isL1ButtonPressed =
     _import "isL1ButtonPressed" public : unit -> int;
+
+  val getR2State =
+    _import "getR2State" public : unit -> Real32.real;
+  val getL2State =
+    _import "getL2State" public : unit -> Real32.real;
 end