/*
 * Set the current X11 Mask.
 *
 * Note: This method is not available unless Imlib2-Ruby was compiled
 * with X11 support.  You can check the constant Imlib2::X11_SUPPORT to
 * see if X11 support is available.
 *
 * Examples:
 *   context.set_mask mask
 *   context.mask = mask
 *
 */
static VALUE ctx_set_mask(VALUE self, VALUE mask_o) {