Input: flip y-motion to match our usual coordinates (top left is (0, 0))

Shared/Input.cs

@@ -73,10 +73,10 @@ namespace SemiColinGames {
         Motion.X = 1;
       }
       if (up && !down) {
-        Motion.Y = 1;
+        Motion.Y = -1;
       }
       if (down && !up) {
-        Motion.Y = -1;
+        Motion.Y = 1;
       }
 
     }