This exposes significantly more of the previously internal encoding guts.

This is beneficial for its own sake as there always seem to be new and interesting things that are supported by the encoding and GPU pipeline that are not directly exposed by the scene builder.

The secondary purpose is laying the groundwork for scene *de*coding and processing. This will enable us to implement various stages of the pipeline on the CPU.

* remove area clamp in fine (now in #241)
* make DrawTag::info_size() const fn
* document DrawColor rgba field
* change Monoid type parameter to an associated type and describe the additional Default constraint
* consistent parens in PathTag::is_subpath_end
* add comments for 32-bit path segment types in PathTag

* also adds low level encoding functions for the three currently supported brushes