Fix GLTF light import

[aaronfranke]

This fixes #53019, a regression from the changes to the GLTF module in #34193. This PR also fixes the attenuation calculation (it's different from 3.x by a factor of 2048) and adds documentation to the GLTFLight class. 3.x version: #53105

For the record, here's why these values are the way they are. GLTF lights with no range specified behave like physical lights which have infinite range, so we default to INFINITY for the range (but this is later clamped to 4096 if making a node). For the color, default to white, which is the most sensible option, otherwise we'd have black lights by default, which do not lighten things. For the intensity, default to 1.0, since 0.0 means no light. For the spotlight angle, default to an eighth turn (on both sides, so the cone's edges are a quarter turn apart), rather than 0.0, which would mean the spotlight is emitting light in an area of zero size.

[fire]

Lights look ok. The lights having power is easier to see when the preview env is off.

[akien-mga]

Thanks!