Specs don't explain that annotations require a padding when the property group size approaches 255

[LucaMarconato]

Reading the JS neuroglancer code for parsing the properties of annotations, I noticed that due some WebGL limitations, a modulo-4 byte padding is expected when the property group size approaches 255 bytes:

neuroglancer/src/annotation/index.ts

Line 384 in 249f866

nextPropertyGroup();

This is not mentioned in the specs, but I think it should, since otherwise an implementation for writing to the precomputed format may not cover this edge case.