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/CMakeLists.txt

@@ -0,0 +1,13 @@
+set(RESOURCELIBNAME pixelariumresourcelib)
+
+set(RESOURCELIBSRC
+  resource.cpp)
+
+add_library(${RESOURCELIBNAME} STATIC
+  ${RESOURCELIBSRC})
+
+target_link_libraries(${RESOURCELIBNAME}
+  PRIVATE pixelariumimagelib)
+
+target_include_directories(${RESOURCELIBNAME}
+  PRIVATE ${CMAKE_SOURCE_DIR}/lib)