filter/graph: add dump() method

src/filter/graph.rs

@@ -1,5 +1,6 @@
 use std::ptr;
-use std::ffi::CString;
+use std::ffi::{CString, CStr};
+use std::str::from_utf8_unchecked;
 
 use ffi::*;
 use libc::c_int;
@@ -76,6 +77,18 @@ impl Graph {
 		}
 	}
 
+	pub fn dump(&self) -> String {
+		unsafe {
+			let ptr    = avfilter_graph_dump(self.as_ptr(), ptr::null());
+			let cstr   = from_utf8_unchecked(CStr::from_ptr((ptr)).to_bytes());
+			let string = cstr.to_owned();
+
+			av_free(ptr as *mut _);
+
+			string
+		}
+	}
+
 	pub fn input(&mut self, name: &str, pad: usize) -> Result<Parser, Error> {
 		Parser::new(self).input(name, pad)
 	}