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/task/project/loadotio/loadotio.cpp

@@ -94,6 +94,8 @@ bool LoadOTIOTask::Run()
 	} else {
 		// Unknown root, we don't know what to do with this
 		SetError(tr("Unknown OpenTimelineIO root element"));
+		delete project_;
+		project_ = nullptr;
 		return false;
 	}