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>

.github/workflows/cmake.yml

@@ -24,3 +24,7 @@ jobs:
       run: cmake -B ${{github.workspace}}/build -S .
     - name: build
       run: cmake --build ${{github.workspace}}/build
+    - name: test
+      run: |
+        cd ${{github.workspace}}/build
+        ctest -C Debug