Fix Unicode mismatch in win32graph (#1629)

The window class is created using RegisterClassA, which means that
the default window procedure should be DefWindowProcA.

Changes

@@ -44,6 +44,10 @@ Working version
 
 ### Other libraries:
 
+- MPR#7445, GPR#1629: Graphics.open_graph displays the correct window title on
+  Windows again (fault introduced by 4.06 Unicode changes).
+  (David Allsopp)
+
 * GPR#1406: Unix.isatty now returns true in the native Windows ports when
   passed a file descriptor connected to a Cygwin PTY. In particular, compiler
   colors for the native Windows ports now work under Cygwin/MSYS2.

otherlibs/win32graph/open.c

@@ -107,7 +107,7 @@ static LRESULT CALLBACK GraphicsWndProc(HWND hwnd,UINT msg,WPARAM wParam,
                 break;
         }
         caml_gr_handle_event(msg, wParam, lParam);
-        return DefWindowProc(hwnd, msg, wParam, lParam);
+        return DefWindowProcA(hwnd, msg, wParam, lParam);
 }
 
 int DoRegisterClass(void)