[pbr_surface] Load glTF file and render it with PBR shader

[Ybalrid]

Hi! here are the changes since last PR on the sample:

This modify the PBR example to render a single mesh from a gltf file. Using the Damaged Sci-Fi Helmet model for testing.

• add "JSON for Modern C++" and tiny_gltf as common libraries for examples
• adapted the gltf_loader from last-time (probably found a bug in UV loading, will PR tinygltf "raytrace" NanoSG example soon to fix)
• fixed face-varying normal interpolation in render loop (linear interpolate like in example from scratchpixels)
• added a way to specify if UVs are wrapped around or clamped on a Sampler2D
• added a way to rotate camera in main() by setting a rotation matrix
• only load-in by hand brdfLUT and environment map textures. Model textures are loaded by tiny_gltf
• added a sample render on readme file

known issue with the example:

• Seems my environment map sampling is off, reflections on curved surfaces shows tearing on the reflected image from cubemap. Need to investigate.

[syoyo]

Good! merged.