Load glTF

[wbrbr]

This PR adds support for the glTF 2.0 file format.

Some features:

• works with .gltf and .glb
• works with embedded (base64) or external textures
• loads the albedo/diffuse texture
• works with multiple mesh per model and multiple primitives per model

Some restrictions (not exhaustive):

• only triangle meshes
• it doesn't support node hierarchies or transforms
• it only loads the diffuse texture. It wouldn't be too hard to make it load other textures (normal, roughness/metalness) if you want to.
• it only supports unsigned short for indices (no byte/unsigned int) and float for texture coordinates (no byte/unsigned short). All the models I have seen loaded correctly but it might still be an issue

Sponza: (there are some rendering artifacts but it loads correctly)

Duck:

[raysan5]

@wbrbr this is an amazing addition! Thank you vey much! I'm merging it as soon as possible! :D

[Leinnan]

How about anims?

[raysan5]

@Leinnan supporting animations requires some extra work... maybe in a future...