Allow the application to draw while Windows is in a modal move/resize loop

If you're using the application main callbacks, your SDL_AppIterate() function will be called while Windows is moving and resizing your window. If not, then SDL will send an SDL_EVENT_WINDOW_EXPOSED event for your window and you can use an event watcher to redraw your window directly from the callback.

Fixes https://github.com/libsdl-org/SDL/issues/1059
Closes https://github.com/libsdl-org/SDL/pull/4836

src/video/windows/SDL_windowsevents.c

@@ -28,6 +28,7 @@
 #include "../../events/SDL_events_c.h"
 #include "../../events/SDL_touch_c.h"
 #include "../../events/scancodes_windows.h"
+#include "../../main/SDL_main_callbacks.h"
 
 /* Dropfile support */
 #include <shellapi.h>
@@ -106,6 +107,10 @@
 #define IS_SURROGATE_PAIR(h, l) (IS_HIGH_SURROGATE(h) && IS_LOW_SURROGATE(l))
 #endif
 
+#ifndef USER_TIMER_MINIMUM
+#define USER_TIMER_MINIMUM 0x0000000A
+#endif
+
 /* Used to compare Windows message timestamps */
 #define SDL_TICKS_PASSED(A, B) ((Sint32)((B) - (A)) <= 0)
 
@@ -1283,6 +1288,31 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
         }
     } break;
 
+    case WM_ENTERSIZEMOVE:
+    case WM_ENTERMENULOOP:
+    {
+        SetTimer(hwnd, (UINT_PTR)SDL_IterateMainCallbacks, USER_TIMER_MINIMUM, NULL);
+    } break;
+
+    case WM_TIMER:
+    {
+        if (wParam == (UINT_PTR)SDL_IterateMainCallbacks) {
+            if (SDL_HasMainCallbacks()) {
+                SDL_IterateMainCallbacks(SDL_FALSE);
+            } else {
+                // Send an expose event so the application can redraw
+                SDL_SendWindowEvent(data->window, SDL_EVENT_WINDOW_EXPOSED, 0, 0);
+            }
+            return 0;
+        }
+    } break;
+
+    case WM_EXITSIZEMOVE:
+    case WM_EXITMENULOOP:
+    {
+        KillTimer(hwnd, (UINT_PTR)SDL_IterateMainCallbacks);
+    } break;
+
     case WM_SIZE:
     {
         switch (wParam) {