All data should be properly initialized by now

They are not automatically unloaded when unloading the model!

When using FormatText() several times in same function, returned string is static and so, the same is returned, resulting in failures on shader loading.

Some tweaks

Issue was related to vertex tangent attibutes not uploaded to GPU, a quick solution was implemented for new vertex attributes loading for already existing meshes... I don't like it specially but it will work for now.

Fixes: c3b948b1 ("Replaced skybox HDR image")

- Renamed Begin3dMode() --> BeginMode3D()
- Renamed Begin2dMode() --> BeginMode2D()
- Renamed End3dMode() --> EndMode3D()
- Renamed End2dMode() --> EndMode2D()

- Added Vector3OrthoNormalize() to raymath.h - not sure if it is correct
- Implemented MeshBinormals() - Mesh struct has not a place for them...
- Updated model_material_pbr example - tested but not working on my GPU
(old Intel HD), actually, it never worked on it...

...and reviewed some details

SetMaterialTexture() and UnsetMaterialTexture()... too shader
dependant...

Moved PBR material loading to example, right decision?