filter::Source: add close method

Expose av_buffersrc_close.

src/filter/context/source.rs

@@ -31,4 +31,13 @@ impl<'a> Source<'a> {
     pub fn flush(&mut self) -> Result<(), Error> {
         unsafe { self.add(&Frame::wrap(ptr::null_mut())) }
     }
+
+    pub fn close(&mut self, pts: i64) -> Result<(), Error> {
+        unsafe {
+            match av_buffersrc_close(self.ctx.as_mut_ptr(), pts, 0) {
+                0 => Ok(()),
+                e => Err(Error::from(e)),
+            }
+        }
+    }
 }