Makes some cases less consistent (and some convenience shortcuts
impossible), but goes well with the attitude "don't use pointer when it
can't be null".
The access methods assert that the user is querying only available data.
Also updated Primitives implementation to create MeshData when
everything is done, not creating empty MeshData and then shooting the
data through interface intended for end users.
* 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.