add solid to list of geometric primitives

[cportele]

to support 3D geometries; closes #4

[jerstlouis]

For 3D content, I see at least 3 types of content which may require slightly different types of styling properties:

• 2D primitives extruded into solids (symbolizers applied to 2D primitives turning them into solids)
• 3D geometry defined explicitly as a solid (or implicitly from 3D MultiPolygons) -- I think both of these are this new 'solid'
• 3D models / meshes with full texture coordinates, normals and other vertex attributes defined, with potentially cusom fragment & vertex shaders, either using legacy Phong reflection model or physical based rendering (e.g. glTF models) -- Styling these I think is a lot more complex, as often the model can define everything to render it, but additional effects and tweaks could be done by a symbolizer.

[jeffharrison]

17 June 2021 SWG Mtg discussed this PR. Comment made to consider adding 'model' to the list. Model will be added before Merging. NOTUC.