Fix #116618: Win32 Use Null for Background Window Brush

Using a non-null background brush does remove an initial white flash
while the program is loading and before we start painting. But this
results in some extra and unnecessary redraws. Default WM_ERASEBKGND
behaviour is to do nothing if this is brush is null, so if non-null we
get erased and must redraw. Suppressing WM_ERASEBKGND will not give us
that initial paint, so no benefit in keeping the brush added in #115968.

Pull Request: https://projects.blender.org/blender/blender/pulls/116642

intern/ghost/intern/GHOST_SystemWin32.cc

@@ -603,7 +603,7 @@ GHOST_TSuccess GHOST_SystemWin32::init()
 #ifdef INW32_COMPISITING
         (HBRUSH)CreateSolidBrush
 #endif
-        (HBRUSH) GetStockObject(DKGRAY_BRUSH);
+        (0x00000000);
     wc.lpszMenuName = 0;
     wc.lpszClassName = L"GHOST_WindowClass";