Some drag'and'drop improvements.

First: disable d'n'd events by default; most apps don't need these at all, and
if an app doesn't explicitly handle these, each drop on the window will cause
a memory leak if the events are enabled. This follows the guidelines we have
for SDL_TEXTINPUT events already.

Second: when events are enabled or disabled, signal the video layer, as it
might be able to inform the OS, causing UI changes or optimizations (for
example, dropping a file icon on a Cocoa app that isn't accepting drops will
cause macOS to show a rejection animation instead of the drop operation just
vanishing into the ether, X11 might show a different cursor when dragging
onto an accepting window, etc).

Third: fill in the drop event details in the test library and enable the
events in testwm.c for making sure this all works as expected.

src/video/windows/SDL_windowsvideo.c

@@ -164,6 +164,7 @@ WIN_CreateDevice(int devindex)
     device->DestroyWindowFramebuffer = WIN_DestroyWindowFramebuffer;
     device->OnWindowEnter = WIN_OnWindowEnter;
     device->SetWindowHitTest = WIN_SetWindowHitTest;
+    device->AcceptDragAndDrop = WIN_AcceptDragAndDrop;
 
     device->shape_driver.CreateShaper = Win32_CreateShaper;
     device->shape_driver.SetWindowShape = Win32_SetWindowShape;

src/video/windows/SDL_windowswindow.c

@@ -289,9 +289,6 @@ SetupWindowData(_THIS, SDL_Window * window, HWND hwnd, HWND parent, SDL_bool cre
         videodata->RegisterTouchWindow(hwnd, (TWF_FINETOUCH|TWF_WANTPALM));
     }
 
-    /* Enable dropping files */
-    DragAcceptFiles(hwnd, TRUE);
-
     data->initializing = SDL_FALSE;
 
     /* All done! */
@@ -979,6 +976,13 @@ WIN_SetWindowOpacity(_THIS, SDL_Window * window, float opacity)
     return 0;
 }
 
+void
+WIN_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept)
+{
+    const SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+    DragAcceptFiles(data->hwnd, accept ? TRUE : FALSE);
+}
+
 #endif /* SDL_VIDEO_DRIVER_WINDOWS */
 
 /* vi: set ts=4 sw=4 expandtab: */

src/video/windows/SDL_windowswindow.h

@@ -78,6 +78,7 @@ extern SDL_bool WIN_GetWindowWMInfo(_THIS, SDL_Window * window,
 extern void WIN_OnWindowEnter(_THIS, SDL_Window * window);
 extern void WIN_UpdateClipCursor(SDL_Window *window);
 extern int WIN_SetWindowHitTest(SDL_Window *window, SDL_bool enabled);
+extern void WIN_AcceptDragAndDrop(SDL_Window * window, SDL_bool accept);
 
 #endif /* SDL_windowswindow_h_ */