Remove superfluous std::sort from atlas generation

[geneotech]

Description

The rectpack2D::find_best_packing function does not in fact alter the input container's order.
This is the only place the actual subjects argument is referenced in the rectpack2D::find_best_packing definition:

			auto& initial_pointers = orders[0];
			initial_pointers.clear();

			for (auto& s : subjects) {
				auto& r = s.get_rect();

				if (r.area() > 0) {
					initial_pointers.emplace_back(std::addressof(r));
				}
			}

The library allocates its own order arrays for each comparator, and manipulates these arrays, not the input container.
Since the input std::vector< QgsTextureRect > rects in generateAtlas is already generated using index++ for every subsequent element, there seems to be no need to ever sort these elements.

[nyalldawson]

Can you also remove the id member from QgsTextureRect? That shouldn't be required anymore.

[github-actions]

🪟 Windows Qt6 builds

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[nyalldawson]

Thanks @geneotech !