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/render/rendermanager.h

@@ -63,11 +63,11 @@ private:
 
 	bool cancelled_;
 
-	Renderer *context_;
+	Renderer *context_ = nullptr;
 
-	DecoderCache *decoder_cache_;
+	DecoderCache *decoder_cache_ = nullptr;
 
-	ShaderCache *shader_cache_;
+	ShaderCache *shader_cache_ = nullptr;
 };
 
 class RenderWorkerPool;
@@ -239,21 +239,21 @@ private:
 
 	static RenderManager *instance_;
 
-	Renderer *context_;
+	Renderer *context_ = nullptr;
 
 	Backend backend_;
 	Backend requested_backend_;
 
-	DecoderCache *decoder_cache_;
+	DecoderCache *decoder_cache_ = nullptr;
 
-	ShaderCache *shader_cache_;
+	ShaderCache *shader_cache_ = nullptr;
 
 	static constexpr auto kDecoderMaximumInactivityAggressive = 1000;
 	static constexpr auto kDecoderMaximumInactivity = 5000;
 
-	int aggressive_gc_;
+	int aggressive_gc_ = 0;
 
-	QTimer *decoder_clear_timer_;
+	QTimer *decoder_clear_timer_ = nullptr;
 
 	RenderThread *dry_run_thread_ = nullptr;
 	RenderThread *audio_thread_ = nullptr;
@@ -263,9 +263,9 @@ private:
 
 	std::list<RenderThread *> render_threads_;
 
-	PreviewAutoCacher *auto_cacher_;
+	PreviewAutoCacher *auto_cacher_ = nullptr;
 
-	RenderWorkerPool *worker_pool_;
+	RenderWorkerPool *worker_pool_ = nullptr;
 
 private slots:
 	void ClearOldDecoders();