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/render_misc_test.cpp

@@ -712,3 +712,22 @@ TEST_F(RenderMiscAutoCacherTest, CacheProxyTaskCancelledClearsPendingJobs)
 
 	cacher.SetProject(nullptr);
 }
+
+// With an unknown/dummy graphics backend the RenderManager never creates the
+// GPU-side objects. Those pointers must be null (previously they were left
+// uninitialized, so callers such as ViewerWidget dereferenced garbage and
+// crashed).
+TEST(RenderManagerDummyBackend, GpuMembersAreNullRatherThanUninitialized)
+{
+	const QVariant previous =
+		olive::Config::Current()[QStringLiteral("GraphicsBackend")];
+	olive::Config::Current()[QStringLiteral("GraphicsBackend")] =
+		QStringLiteral("dummy");
+
+	olive::RenderManager::CreateInstance();
+
+	EXPECT_EQ(olive::RenderManager::instance()->GetCacher(), nullptr);
+
+	olive::RenderManager::DestroyInstance();
+	olive::Config::Current()[QStringLiteral("GraphicsBackend")] = previous;
+}