Improve EXT_mesh_gpu_instancing validation

[zeux]

cgltf_validate will now check that EXT_mesh_gpu_instancing nodes adhere to basic contracts:

• The mesh must be present on the node; if absent, an implementation is likely to accidentally dereference a NULL pointer
• Attributes must have consistent counts; this is critical because the number of instances is implicitly specified via accessor counts and if any of them diverge, an out of bounds access is likely to occur