FEATURE: Extend DTX to support "lines" primitives

[xeolabs]

Extend data texture scene representation (DTX) to support lines primitives.

This way we can use DTX for everything in an IFC model, while also leveraging DTX's superior memory footprint for models that have zillions of line segments.

This enables us to efficiently use xeokit's new dynamic transforms to animate objects made of line segments.

Before this, line segments can only be represented as instanced or batched VBOs. Instanced VBOs are inefficient for large numbers of objects, while batched VBOs cannot have dynamic transforms, because the transform matrix needs to be duplicated per-vertex. Therefore, DTX is the only efficient way to have large numbers of dynamically transformed objects, which applies to line segments as well as triangles.

Tested using the ArchiCAD test model - see the dimension lines in the snapshot below.