otio: write importable OTIO/FCPXML (DaVinci Resolve rejects both)

- available_range declared a zero duration for every clip (the media
  was 'unusable', so Resolve invalidated all of them): it now covers
  the whole media - video clips in the probed frame rate, audio clips
  in the probed sample rate (no more hardcoded 48000/0).
- Gap ranges mixed time bases (a 24 fps block start, a 1.0 trailing
  duration): gaps now stay in the sequence's time base, the trailing
  pad in whole frames.
- Empty sequences (no clips) are skipped, so a lone edited sequence
  exports as a bare Timeline root instead of a SerializableCollection
  Resolve cannot read.
- FCPXML: an asset used on both video and audio tracks now declares
  hasVideo AND hasAudio (the first registration's track kind used to
  hide the other stream); asset durations recover with available_range.

crates/oak-otio/src/fcpxml.rs

@@ -953,6 +953,15 @@ impl ResourceSet {
 				};
 				let src = external.target_url();
 				if self.asset_ids.contains_key(src) {
+					// Already registered from another track: union the
+					// stream flags so a file used on both a video and an
+					// audio track declares hasVideo AND hasAudio (the
+					// first registration's track kind must not hide the
+					// other stream from the importer).
+					if let Some(existing) = self.assets.iter_mut().find(|a| a.src == src) {
+						existing.has_video |= is_video;
+						existing.has_audio |= is_audio;
+					}
 					continue;
 				}
 				let (asset_format_id, asset_duration) = match external.available_range() {