Add a Multi-Segments element

In recent versions of datashader there are much more efficient means of drawing fixed length segments, however we have to way of representing that data in HoloViews. I therefore propose we add a Segments element where the x-/y- coordinates are stored as key dimensions. This can also be used to render a TriMesh wireframe (indeed internally the trimesh_rasterize operation already does this). Once added I’d therefore create a public property on TriMesh which makes the wireframe available as a Segments element.

[philippjfr]

Segments were added.

[poplarShift]

In that case I’d prefer alternating - it would be easier to modify the number of nodes, just clipping/pasting in one place instead of at least two per node.