UI: Remove Initial Win32 White Flash During Startup

On the Windows platform we get an initial bright white flash during
startup, before we start drawing on our canvas. This PR changes that
initial color to a mid-grey that is much less noticeable. After this
initial fill is done we then clear the background brush so Windows will
not use it later and slow down resizing by adding extra WM_PAINT
messages and fills.

Pull Request: https://projects.blender.org/blender/blender/pulls/137488

intern/ghost/intern/GHOST_SystemWin32.cc

@@ -622,7 +622,7 @@ GHOST_TSuccess GHOST_SystemWin32::init()
 #ifdef INW32_COMPISITING
         (HBRUSH)CreateSolidBrush
 #endif
-        (0x00000000);
+        (HBRUSH) GetStockObject(DKGRAY_BRUSH);
     wc.lpszMenuName = 0;
     wc.lpszClassName = L"GHOST_WindowClass";
 
@@ -2245,6 +2245,18 @@ LRESULT WINAPI GHOST_SystemWin32::s_wndProc(HWND hwnd, uint msg, WPARAM wParam,
           /* An application sends the WM_ERASEBKGND message when the window background must be
            * erased (for example, when a window is resized). The message is sent to prepare an
            * invalidated portion of a window for painting. */
+          {
+            HBRUSH bgBrush = (HBRUSH)GetClassLongPtr(hwnd, GCLP_HBRBACKGROUND);
+            if (bgBrush) {
+              RECT rect;
+              GetClientRect(hwnd, &rect);
+              FillRect((HDC)(wParam), &rect, bgBrush);
+              /* Clear the backround brush after the initial fill as we don't
+               * need or want any default Windows fill behavior on redraw. */
+              SetClassLongPtr(hwnd, GCLP_HBRBACKGROUND, (LONG_PTR) nullptr);
+            }
+            break;
+          }
         case WM_NCPAINT:
           /* An application sends the WM_NCPAINT message to a window
            * when its frame must be painted. */