Only force the resize event in the DPI changed case

OpenGL windows don't actually get the WM_WINDOWPOSCHANGED event in the SetWindowPos() call in WIN_SetWindowFullscreen(), so setting the window size to zero never gets reset and we're stuck with a zero sized window.

Instead, just force the resize event in WM_DPICHANGED handling, where we know we need it. If we end up needing to force it in WIN_SetWindowFullscreen(), just set a flag in the window data and respond to that in WM_WINDOWPOSCHANGED, but that's a fairly risky behavior change as suddenly all applications would start getting SDL_WINDOWEVENT_SIZE_CHANGED when going fullscreen, and they may respond to that in expensive and potentially disruptive ways.

For later we'll probably create a DPI changed event and respond to that in the renderer instead of this window size changed hack.

This fixes https://github.com/libsdl-org/SDL/issues/6033 @ericwa

src/video/windows/SDL_windowsevents.c

@@ -1548,7 +1548,14 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                     data->scaling_dpi = newDPI;
 
                     /* Send a SDL_WINDOWEVENT_SIZE_CHANGED saying that the client size (in dpi-scaled points) is unchanged.
-                       Renderers need to get this to know that the framebuffer size changed. */
+                       Renderers need to get this to know that the framebuffer size changed.
+
+                       We clear the window size to force the event to be delivered, but what we really
+                       want for SDL3 is a new event to notify that the DPI changed and then watch for
+                       that in the renderer directly.
+                     */
+                    data->window->w = 0;
+                    data->window->h = 0;
                     SDL_SendWindowEvent(data->window, SDL_WINDOWEVENT_SIZE_CHANGED, data->window->w, data->window->h);
                 }
 

src/video/windows/SDL_windowswindow.c

@@ -932,11 +932,6 @@ WIN_SetWindowFullscreen(_THIS, SDL_Window * window, SDL_VideoDisplay * display,
     SDL_Log("WIN_SetWindowFullscreen: %d", (int)fullscreen);
 #endif
 
-    /* Clear the window size, to force SDL_SendWindowEvent to send a SDL_WINDOWEVENT_RESIZED
-       event in WM_WINDOWPOSCHANGED. */
-    data->window->w = 0;
-    data->window->h = 0;
-
     if (SDL_ShouldAllowTopmost() && (window->flags & SDL_WINDOW_ALWAYS_ON_TOP)) {
         top = HWND_TOPMOST;
     } else {