/*
* 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) {