codec/decoder: add time_base() method

src/codec/decoder/mod.rs

@@ -24,7 +24,7 @@ use std::ops::{Deref, DerefMut};
 
 use ffi::*;
 use super::{Id, Context};
-use ::{Codec, Error, Discard, Dictionary};
+use ::{Codec, Error, Discard, Rational, Dictionary};
 
 pub struct Decoder(pub Context);
 
@@ -127,6 +127,12 @@ impl Decoder {
 			(*self.as_mut_ptr()).skip_frame = value.into();
 		}
 	}
+
+	pub fn time_base(&self) -> Rational {
+		unsafe {
+			Rational::from((*self.as_ptr()).time_base)
+		}
+	}
 }
 
 impl Deref for Decoder {