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/ScreenSettings.h

@@ -1,6 +1,14 @@
 #ifndef SCREENSETTINGS_H
 #define SCREENSETTINGS_H
 
+enum
+{
+    SCALING_LETTERBOX = 0,
+    SCALING_STRETCH = 1,
+    SCALING_INTEGER = 2,
+    NUM_SCALING_MODES
+};
+
 struct ScreenSettings
 {
     int windowWidth;