Separate model loading from reference renderer

[hoffstadt]

Purpose

The purpose of this pull request is to separate model loading from the reference renderer. This was accomplished by creating a new extension.

Other

This PR also makes memory tracking safe.

API

//-----------------------------------------------------------------------------
// [SECTION] public api structs
//-----------------------------------------------------------------------------

typedef struct _plModelLoaderI
{
    bool (*load_stl) (plComponentLibrary* ptLibrary, const char* pcPath, plVec4 tColor, const plMat4* ptTransform, plModelLoaderData* ptDataOut);
    bool (*load_gltf)(plComponentLibrary* ptLibrary, const char* pcPath, const plMat4* ptTransform, plModelLoaderData* ptDataOut);

    void (*free_data)(plModelLoaderData*);
} plModelLoaderI;

//-----------------------------------------------------------------------------
// [SECTION] structs
//-----------------------------------------------------------------------------

typedef struct _plModelLoaderData
{
    uint32_t  uOpaqueCount;
    plEntity* atOpaqueObjects;
    uint32_t  uTransparentCount;
    plEntity* atTransparentObjects;
} plModelLoaderData;