Interpolate end picture reveal scroll

This is so it looks smooth at framerates above 30.

desktop_version/src/Game.cpp

@@ -162,6 +162,7 @@ void Game::init(void)
     creditposx = 0;
     creditposy = 0;
     creditposdelay = 0;
+    oldcreditposx = 0;
 
     useteleporter = false;
     teleport_to_teleporter = 0;

desktop_version/src/Game.h

@@ -232,6 +232,7 @@ public:
     enum Menu::MenuName menudest;
 
     int creditposx, creditposy, creditposdelay;
+    int oldcreditposx;
 
 
     //Sine Wave Ninja Minigame

desktop_version/src/Input.cpp

@@ -2220,10 +2220,13 @@ void gamecompleteinput2()
     game.press_action = false;
     game.press_map = false;
 
+    //Do this here because input comes first
+    game.oldcreditposx = game.creditposx;
 
     if (key.isDown(KEYBOARD_z) || key.isDown(KEYBOARD_SPACE) || key.isDown(KEYBOARD_v) || key.isDown(game.controllerButton_flip))
     {
         game.creditposx++;
+        game.oldcreditposx++;
         if (game.creditposy >= 30)
         {
             if(graphics.fademode==0)

desktop_version/src/Render.cpp

@@ -1272,6 +1272,8 @@ void gamecompleterender2()
         }
     }
 
+    FillRect(graphics.backBuffer, graphics.lerp(game.oldcreditposx * 8, game.creditposx * 8) + 8, game.creditposy * 8, 8, 8, 0, 0, 0);
+
     graphics.drawfade();
 
     graphics.render();