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/*
 * Apply a scripted filter or a static (eg Imlib2::Filter) filter 
 *
 * Example:
 *   # apply a static filter
 *   filter = Imlib2::Filter.new 20
 *   filter.set 2, 2, Imlib2::Color::GREEN
 *   image.filter filter
 *
 *   # apply a scripted filter
 *   x, y = 20, 10
 *   filter_string = "tint( x=#{x}, y=#{y}, red=255, alpha=55 );"
 *   image.filter filter_string
 *
 */
static VALUE image_filter(VALUE self, VALUE filter) {