/*
 * Apply a scripted filter
 *
 * You should probably using Imlib2::Image#filter() instead, since it is
 * polymorphic (eg, it can handle both static and scripted filters).
 * 
 * Example:
 *   x, y = 20, 10
 *   filter_string = "tint( x=#{x}, y=#{y}, red=255, alpha=55 );"
 *   image.script_filter filter_string
 *
 */
static VALUE image_script_filter(VALUE self, VALUE filter) {