Support KHR_animation_pointer

[mklefrancois]

The extension KHR_animation_pointer provides a path to the element to animate, and the path might be like this: /materials/0/pbrMetallicRoughness/baseColorFactor

Unfortunately, parsing such string is rather difficult in tinygltf without a direct access to JSON.

Would it be possible to provide a function, like void* resolvePointerPath(const std::string& pointerPath) which would return a pointer to the element?

This scene from the official glTF-Asset can be used for testing: https://github.com/KhronosGroup/glTF-Sample-Assets/tree/main/Models/AnimatedColorsCube

[syoyo]

You can contribute writing a function to parse string in KHR_animation_pointer.

PR is much appreciated!