Add and use scaling mode enum

This enum is to just make each mode be readable, instead of mysterious
0/1/2 values. It's not a strictly-typed enum because we still have to
serialize it as ints in the XML, but it's better than just leaving them
as ints.

This also adds a NUM_SCALING_MODES enum, so we don't have to hardcode
that 3 when cycling scaling modes anymore.

desktop_version/src/Render.cpp

@@ -318,12 +318,13 @@ static void menurender(void)
 
             switch (gameScreen.scalingMode)
             {
-            case 2:
+            case SCALING_INTEGER:
                 graphics.Print( -1, 85, "Current mode: INTEGER", tr, tg, tb, true);
                 break;
-            case 1:
+            case SCALING_STRETCH:
                 graphics.Print( -1, 85, "Current mode: STRETCH", tr, tg, tb, true);
                 break;
+            case SCALING_LETTERBOX:
             default:
                 graphics.Print( -1, 85, "Current mode: LETTERBOX", tr, tg, tb, true);
                 break;