Update the Renderer dpi_scale on SIZE_CHANGED event (fix #4580)

The Renderer logical scaling code scales mouse coordinates, and needs to
take the window DPI into account on HIGHDPI windows. However, the
variable which tracks this, renderer->dpi_scale, is set once when the
renderer is created, and then not updated. In the event that the window
is moved to another screen, or the screen DPI otherwise changes, this
will be outdates, and potentially the coordinates will be all wrong.

So let's update the dpi_scale on the SIZE_CHANGED event: it's at least a
possibility that this will be issued on some OSes when DPI changes, and
it's otherwise already handled by SDL_Renderer's event filter.

src/render/SDL_render.c

@@ -619,6 +619,17 @@ SDL_RendererEventWatch(void *userdata, SDL_Event *event)
                     SDL_SetRenderTarget(renderer, NULL);
                 }
 
+                /* Update the DPI scale if the window has been resized. */
+                if (window && renderer->GetOutputSize) {
+                    int window_w, window_h;
+                    int output_w, output_h;
+                    if (renderer->GetOutputSize(renderer, &output_w, &output_h) == 0) {
+                        SDL_GetWindowSize(renderer->window, &window_w, &window_h);
+                        renderer->dpi_scale.x = (float)window_w / output_w;
+                        renderer->dpi_scale.y = (float)window_h / output_h;
+                    }
+                }
+
                 if (renderer->logical_w) {
                     UpdateLogicalSize(renderer);
                 } else {