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>

modules/googletest.cmake

@@ -0,0 +1,13 @@
+include(FetchContent)
+FetchContent_Declare(
+  googletest
+  URL https://github.com/google/googletest/archive/refs/tags/v1.14.0.zip
+)
+# From this documentation: https://google.github.io/googletest/quickstart-cmake.html
+# For Windows: Prevent overriding the parent project's compiler/linker settings
+
+if(WIN32)
+  set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+endif()
+
+FetchContent_MakeAvailable(googletest)