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

tests/gtest/widget_timeruler_playback_test.cpp

@@ -100,6 +100,20 @@ TEST(TimeRuler, SeekToScenePointWithoutTimebaseIsNoOp)
 	SUCCEED();
 }
 
+TEST(TimeRuler, SeekToScenePointWithoutViewerIsNoOp)
+{
+	EnsureAppSingletons();
+
+	// Timebase set but no viewer connected: previously dereferenced a null
+	// GetViewerNode() and crashed.
+	TimeRuler ruler;
+	ruler.SetTimebase(rational(1, 30));
+	ruler.SetScale(100.0);
+
+	ruler.SeekToScenePoint(150.0);
+	SUCCEED();
+}
+
 class PlaybackControlsTest : public ::testing::Test {
 protected:
 	void SetUp() override