Add Sink::time_base

Calls `av_buffersink_get_time_base`.

src/filter/context/sink.rs

@@ -1,7 +1,7 @@
 use super::Context;
 use ffi::*;
 use libc::c_int;
-use {Error, Frame};
+use {Error, Frame, Rational};
 
 pub struct Sink<'a> {
     ctx: &'a mut Context<'a>,
@@ -41,4 +41,8 @@ impl<'a> Sink<'a> {
             av_buffersink_set_frame_size(self.ctx.as_mut_ptr(), value);
         }
     }
+
+    pub fn time_base(&self) -> Rational {
+        unsafe { av_buffersink_get_time_base(self.ctx.as_ptr()) }.into()
+    }
 }