Win32: Clear to black in WM_PAINT for transparent framebuffer windows

The DWM redirection surface may contain undefined alpha values,
causing the window to composite incorrectly. PatBlt with BLACKNESS
clears both RGB and alpha to zero.

src/win32_window.c

@@ -1139,6 +1139,22 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM l
         case WM_PAINT:
         {
             _glfwInputWindowDamage(window);
+
+            // The DWM redirection surface starts with undefined alpha.
+            // Clear to BLACKNESS which should zero out the alpha components as well.
+            if (window->win32.transparent)
+            {
+                PAINTSTRUCT ps;
+                HDC hdc = BeginPaint(hWnd, &ps);
+                PatBlt(hdc,
+                       ps.rcPaint.left,
+                       ps.rcPaint.top,
+                       ps.rcPaint.right  - ps.rcPaint.left,
+                       ps.rcPaint.bottom - ps.rcPaint.top,
+                       BLACKNESS);
+                EndPaint(hWnd, &ps);
+                return 0;
+            }
             break;
         }