fix: remaining issues recorded during test coverage work

- RenderManager: GPU-side members (context_, decoder_cache_,
  shader_cache_, auto_cacher_, worker_pool_, decoder_clear_timer_) were
  left uninitialized when the configured graphics backend is unknown
  (e.g. dummy); ViewerWidget then dereferenced garbage and crashed.
  Initialize them at declaration
- CurveView::SelectKeyframesOfInput ignored its reference parameter and
  selected keyframes of every connected track; select only the
  requested track's keyframes
- SeekableWidget::SeekToScenePoint dereferenced GetViewerNode()
  unconditionally; skip the playhead update when no viewer is connected
- LoadOTIOTask: unknown root schema leaked the freshly allocated
  project_ (delete + reset; OTIO is not enabled in local builds so this
  file is compile-verified by inspection only)

Locked by new tests: RenderManagerDummyBackend,
TimeRuler.SeekToScenePointWithoutViewerIsNoOp,
CurveViewTest.SelectKeyframesOfInputSelectsOnlyRequestedTrack

app/widget/curvewidget/curveview.cpp

@@ -91,7 +91,7 @@ void CurveView::SelectKeyframesOfInput(const NodeKeyframeTrackReference &ref)
 {
 	DeselectAll();
 
-	foreach (KeyframeViewInputConnection *con, track_connections_) {
+	if (KeyframeViewInputConnection *con = track_connections_.value(ref)) {
 		foreach (NodeKeyframe *key, con->GetKeyframes()) {
 			SelectKeyframe(key);
 		}

app/widget/timeruler/seekablewidget.cpp

@@ -433,8 +433,9 @@ void SeekableWidget::SeekToScenePoint(qreal scene)
 		playhead_time += movement;
 	}
 
-	if (playhead_time != GetViewerNode()->GetPlayhead()) {
-		GetViewerNode()->SetPlayhead(playhead_time);
+	ViewerOutput *viewer = GetViewerNode();
+	if (viewer && playhead_time != viewer->GetPlayhead()) {
+		viewer->SetPlayhead(playhead_time);
 	}
 }