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/imaging/PixelariumImage.cpp

@@ -0,0 +1,17 @@
+#include "PixelariumImage.hpp"
+
+#include <filesystem>
+#include <format>
+#include <memory>
+#include <opencv2/imgcodecs.hpp>
+#include <stdexcept>
+
+pixelarium::imaging::PixelariumImage::PixelariumImage(const std::string& uri)
+{
+    if (!std::filesystem::exists(uri))
+    {
+        throw std::runtime_error(std::format("File not {} found", uri));
+    }
+
+    this->img_ = std::make_unique<cv::Mat>(cv::imread(uri));
+}