wined3d: Use glFinish() for synchronisation when cleaning up a destroyed context in wined3d_context_gl_cleanup().

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=50485Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>