Minor cleanup in classes vkb[::core::HPP]::Swapchain

[asuessenbach]

Description

Fixes #972

Build tested on Win10 with VS2022. Run tested on Win10 with NVidia GPU.

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[SaschaWillems]

Stupid question, but is the non-hpp swapchain.cpp actually used anymore? I'm working on a PR that changes a few minor things in the swapchain units (among others) and even the non-hpp api samples end up using the hpp_swapchain.cpp instead? Not sure why, or if that's intentional at all.

[asuessenbach]

Indeed, the Swapchain isn't used that often any more. The "unified" class VulkanSample holds a vkb::core::HPPSwapchain as a member, no matter whether it's providing the C- or the C++-bindings-based interface. Therefore, all the samples effectively use the HPPSwapchain now. But there's a function VulkanSample::get_render_context, that depending on the used binding returns a RenderContext or an HPPRenderContext, and via that route, you can also get a Swapchain. And there are a couple of classes, that actually use a RenderContext (All the PostProcessing* classes, for example), and thus a Swapchain could still be used.