/*
 * Get the current X11 Drawable.
 *
 * 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:
 *   drawable = context.get_drawable
 *   drawable = context.drawable
 *
 */
static VALUE ctx_drawable(VALUE self) {