format/context: implement DerefMut

src/format/context.rs

@@ -2,7 +2,7 @@ use std::ffi::CString;
 use std::ptr;
 use std::path::Path;
 use std::marker::PhantomData;
-use std::ops::Deref;
+use std::ops::{Deref, DerefMut};
 
 use libc::c_uint;
 use ffi::*;
@@ -183,6 +183,12 @@ impl<'a> Deref for Packet<'a> {
 	}
 }
 
+impl<'a> DerefMut for Packet<'a> {
+	fn deref_mut<'b>(&'b mut self) -> &'b mut ::Packet {
+		&mut self.pkt
+	}
+}
+
 pub struct StreamIter<'a> {
 	ptr: *const AVFormatContext,
 	cur: c_uint,