[0.4.12][COMCTL32] Workaround v6 button redraw regressions CORE-13278 & CORE-16093

Most likely not correct, as it breaks comctl32:buttonv6 tests
https://reactos.org/testman/compare.php?ids=68146,68147

But greatly improves perceived flickering when hovering
with the mouse about buttons in unthemed mode.
We still fear that this workaround could break buttons that
intend to change their appearance when being hovered.

We workaround only the RC, but will leave master affected for now.

Author: JoachimHenze

dll/win32/comctl32/button.c

@@ -842,7 +842,9 @@ static LRESULT CALLBACK BUTTON_WindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, L
             nmhotitem.dwFlags      = HICF_ENTERING;
             SendMessageW(GetParent(hWnd), WM_NOTIFY, nmhotitem.hdr.idFrom, (LPARAM)&nmhotitem);
 
-            InvalidateRect(hWnd, NULL, TRUE);
+            theme = GetWindowTheme( hWnd );
+            if (theme)
+                InvalidateRect(hWnd, NULL, TRUE);
         }
 
         if(!TrackMouseEvent(&mouse_event) || !(mouse_event.dwFlags&TME_LEAVE))
@@ -896,7 +898,9 @@ static LRESULT CALLBACK BUTTON_WindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, L
             nmhotitem.dwFlags      = HICF_LEAVING;
             SendMessageW(GetParent(hWnd), WM_NOTIFY, nmhotitem.hdr.idFrom, (LPARAM)&nmhotitem);
 
-            InvalidateRect(hWnd, NULL, TRUE);
+            theme = GetWindowTheme( hWnd );
+            if (theme)
+                InvalidateRect(hWnd, NULL, TRUE);
         }
         break;
 #else