Resources (#1)

* start some resource fiddling

* start some resource fiddling

* initiate a real resource manager

* fix color flicker in rendering

* delete unintended constructors

and add a convenience function to reset the render-image (from the
original image, aka. clone again)

* [OpenGL deprecation warning]

The compiler said that these functions are "deprecated". They seem to
be useless anyway...

* various improvements

* add resource enumerator and documentation

* fix constness stuff

* use existing iterator for insertion

* init unit tests

* rm bogus file

---------

Co-authored-by: m-aXimilian <keuffnermax@gmail.com>

lib/resources/resource.hpp

@@ -0,0 +1,58 @@
+#pragma once
+
+#include <functional>
+#include <memory>
+#include <optional>
+#include <unordered_map>
+
+#include "imaging/PixelariumImage.hpp"
+
+namespace pixelarium::resources
+{
+struct IResource
+{
+    virtual ~IResource() = 0;
+};
+
+template <typename R>
+concept ResT = requires(R& r) { static_cast<IResource&>(r); };
+
+// template <ResT R>
+template <typename R>
+class IResourcePool
+{
+   public:
+    virtual ~IResourcePool() = default;
+    virtual std::optional<const R*> GetResource(size_t id) const = 0;
+    virtual size_t SetResource(std::unique_ptr<R> res) = 0;
+    virtual bool UpdateResource(size_t id, std::unique_ptr<R> res) = 0;
+    virtual bool DeleteResource(size_t id) = 0;
+    virtual void EnumerateResources(const std::function<void(size_t, const R&)>& func) = 0;
+};
+
+// Now with the =GetResource= method, I do not want to transfer ownership to the caller of that method. The ownership
+// should still
+// reside with the =ResourcePool=!
+// In fact, the intention is, that there is no way back once the =ResourcePool= took ownership of an object.
+// Callers can get references, but no ownership. A caller might delete a resource though.
+class ImageResourcePool : public IResourcePool<imaging::PixelariumImage>
+{
+   public:
+    ImageResourcePool() = default;
+    ImageResourcePool(ImageResourcePool&) = delete;
+    ImageResourcePool(const ImageResourcePool&) = delete;
+    ImageResourcePool(ImageResourcePool&&) = delete;
+    ImageResourcePool& operator=(ImageResourcePool&) = delete;
+    ImageResourcePool& operator=(ImageResourcePool&&) = delete;
+
+    std::optional<const imaging::PixelariumImage*> GetResource(size_t id) const override;
+    size_t SetResource(std::unique_ptr<imaging::PixelariumImage> res) override;
+    bool UpdateResource(size_t id, std::unique_ptr<imaging::PixelariumImage> res) override;
+    bool DeleteResource(size_t id) override;
+
+    void EnumerateResources(const std::function<void(size_t, const imaging::PixelariumImage&)>& func) override;
+
+   private:
+    std::unordered_map<size_t, std::unique_ptr<imaging::PixelariumImage>> resources_;
+};
+}  // namespace pixelarium::resources