Differentiate between required and optional pointers during pointer fixup
Differentiate between required and optional pointers during pointer fixup

Some of the pointers in GLTF spec are required; most importantly, all
arrays of references to nodes can't have NULL entries - if the input
GLTF contains -1 indices then we'll get NULL nodes.

Introduce a _REQ macro that checks whether a pointer is NULL to address
this.

jkuhlmann/cgltf,jkuhlmann/cgltf,jkuhlmann/cgltf