Fix: Ensure OpenGL context is initialized for GrayscaleFilter

[vlivashkin]

Problem:
GrayscaleFilter was crashing with Segmentation fault during initialization because it was creating OpenGL shaders outside of the OpenGL context. This affected all filters that depend on GrayscaleFilter (like CannyEdgeDetectionFilter).

Fix:
Added SyncRunWithContext in GrayscaleFilter::Create() to ensure shader creation happens in the correct OpenGL context:

std::shared_ptr<GrayscaleFilter> GrayscaleFilter::Create() {
  auto ret = std::shared_ptr<GrayscaleFilter>(new GrayscaleFilter());
  gpupixel::GPUPixelContext::GetInstance()->SyncRunWithContext([&] {
    if (!ret->Init()) {
      ret.reset();
    }
  });
  return ret;
}

Testing:
Both GrayscaleFilter and CannyEdgeDetectionFilter now work correctly.