GLTF loader improvements

[igagis]

For simpler .glb model preparation need to support:

• when metalness/roughness/ambient_occlusion is defined in .glb file as a single value (not texture), need to support that
• combine metalness/roughtness/ambient_occlusion separate textures into a single arm texture if glTF file defines separate textures for those (because PBR shader requires singel arm texture as input). Green channel = roughness, Blue channel = metalness, Red channel = ambient_occlusion (see glTF spec).
• use exported tangents if present