Fix the erroneous generation of mouse-down events from touch-move events.

The issue is that MS Windows synthesizes a mouse-move event in response
to touch-move events, and those mouse-move events are NOT labeled as
coming from a touch (e.g. GetMouseMessageSource() will not return
SDL_MOUSE_EVENT_SOURCE_TOUCH for those synthesized mouse-move events).

In addition, there seems to be no way to prevent this from happening;
https://gist.github.com/vbfox/1339671 claims to demonstrate a technique
to prevent it, but in my experience, it doesn't work.

Because of this, the "fallthrough" case can't test that the synthesized
mouse-move came from a touch-move, and starts erroneously pressing down
the mouse-button, leading to massive confusion in the client
application.

src/video/windows/SDL_windowsevents.c

@@ -732,8 +732,8 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                 }
             }
         }
-        /* don't break here, fall through to check the wParam like the button presses */
-        SDL_FALLTHROUGH;
+        break;
+
     case WM_LBUTTONUP:
     case WM_RBUTTONUP:
     case WM_MBUTTONUP: