Interpolate credits position

So that it's as smooth as possible, especially when holding down ACTION
to make it go really fast.

desktop_version/src/Game.h

@@ -260,6 +260,7 @@ public:
     int timetrialpar, timetrialresulttime, timetrialrank;
 
     int creditposition;
+    int oldcreditposition;
     int creditmaxposition;
     std::vector<const char*> superpatrons;
     std::vector<const char*> patrons;