Add read/write support for KHR_animation_pointer

[emackey]

When reading a glTF, the cgltf_animation_path_type enum (channel.target_path) gets a new possible value of cgltf_animation_path_type_pointer, and alongside that, the animation channel has a new string target_pointer which holds a "JSON pointer" (a string, formatted as a path within glTF's JSON). The KHR_animation_pointer extension, when properly applied to an animation channel, will both set the new enum value and populate the new target_pointer string.

When writing a glTF, if the new enum value is set and target_pointer is not null, the KHR_animation_pointer extension will be written.

Available "JSON pointers" in glTF are described here: The glTF Asset Object Model.

The pointer is stored in an extension specified here: KHR_animation_pointer.

Some sample assets demonstrate this in action: AnimatedColorsCube, AnimationPointerUVs

[emackey]

Is there anything I can do to help this along?