Method GetFrame

tests/EffectMask.cpp:99–104

	void Close() override { is_open = false; }

	std::shared_ptr<openshot::Frame> GetFrame(int64_t number) override {
		requests.push_back(number);
		auto frame = std::make_shared<Frame>(number, width, height, "#00000000");
		frame->GetImage()->fill(QColor(128, 128, 128, 255));
		return frame;
	}

	std::string Json() const override {
		return JsonValue().toStyledString();