dav1ddec: Make sure to call get_picture() twice in a row when draining

The first time might return `EAGAIN` if there are pending frames but
there is no decoded frame available yet. The second time it will
actually wait for frames to become available and only start returning
`EAGAIN` again once no more frames are left.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1080>

video/dav1d/src/dav1ddec/imp.rs

@@ -430,13 +430,25 @@ impl Dav1dDec {
         mut state_guard: MutexGuard<'s, Option<State>>,
         drain: bool,
     ) -> Result<MutexGuard<Option<State>>, gst::FlowError> {
+        // dav1d wants to have get_picture() called a second time after it return EAGAIN to
+        // actually drain all pending pictures.
+        let mut call_twice = drain;
+
+        loop {
             while let Some(pic) = self.pending_picture(&mut state_guard)? {
                 state_guard = self.handle_picture(state_guard, &pic)?;
+                call_twice = false;
                 if !drain {
                     break;
                 }
             }
 
+            if !call_twice {
+                break;
+            }
+            call_twice = false;
+        }
+
         Ok(state_guard)
     }
 }