Cygwin/X: Avoid a visual glitch on window move in rootless modes

Handle and ignore WM_ERASEBKGND since we repaint the entire invalidated region anyhow
(this avoids a white flickering on window resize)

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>

hw/xwin/winmultiwindowwndproc.c

@@ -453,6 +453,14 @@ winTopLevelWindowProc (HWND hwnd, UINT message,
       HandleCustomWM_INITMENU ((unsigned long)hwnd, wParam);
       break;
 
+    case WM_ERASEBKGND:
+      /*
+       * Pretend that we did erase the background but we don't care,
+       * since we repaint the entire region anyhow
+       * This avoids some flickering when resizing.
+       */
+      return TRUE;
+
     case WM_PAINT:
       /* Only paint if our window handle is valid */
       if (hwndScreen == NULL)

hw/xwin/winwin32rootlesswndproc.c

@@ -783,6 +783,17 @@ winMWExtWMWindowProc (HWND hwnd, UINT message,
       SendMessage (hwndScreen, message, wParam, lParam);
       return 0;
 
+    case WM_ERASEBKGND:
+#if CYGDEBUG
+      winDebug ("winMWExtWMWindowProc - WM_ERASEBKGND\n");
+#endif
+      /*
+       * Pretend that we did erase the background but we don't care,
+       * since we repaint the entire region anyhow
+       * This avoids some flickering when resizing.
+       */
+      return TRUE;
+
     case WM_PAINT:
     
       /* BeginPaint gives us an hdc that clips to the invalidated region */