Generalize stretch mode mouse scaling fix

This puts the code to fix mouse coordinates in stretch mode directly
inside KeyPoll::Poll, preventing the need for any other instances of
mouse coordinate usage to copy-paste code.

desktop_version/src/KeyPoll.h

@@ -63,7 +63,8 @@ public:
     bool controllerWantsDown(void);
 
     int leftbutton, rightbutton, middlebutton;
-    int mx, my;
+    int mousex;
+    int mousey;
 
     bool textentry(void);
     bool pressedbackspace;