* Calling enable_testing() only in root path.
* Using CORRADE_CXX_FLAGS instead of our own set to make things easier
to maintain.
* Various cleanup and reorganization.
* Added missing forward declarations.
* Added getters and setters to ShapeRendererOptions class.
* Cube primitive is indexed, modified the mesh creation accordingly. It
now uses MeshTools, link the library to DebugTools.
* Simplified resource handling so it's now handled in one place. It
still needs to be thought out better.