Implement support for KHR_lights_punctual
Implement support for KHR_lights_punctual

This change adds support for parsing light information into lights array
and parsing node->light reference. We parse directional, point and spot
information; spot data is saved into two float variables since it felt
excessive to create a separate struct for this.

Additionally there were a few missing checks in the extension parsing
for other extensions, which this change adds.

jkuhlmann/cgltf,jkuhlmann/cgltf,jkuhlmann/cgltf