Path pattern fills

[alexcjohnson]

Arbitrary fill patterns! For example geologists have all sorts of standardized patterns they know and love, and this lets you create any pattern you want. It still has the constraint of two colors (foreground and background) and a square unit cell, but now any svg path string is accepted to define the foreground color area.

I chose to do this by adding a new attribute in the pattern container, path. We could instead put the path into the shape attribute - in fact we only accept one (shape takes precedence) and for drawing I combine them into one anyway. And this is what we do for line.dash for example, there's an enumerated set but you also have the option of providing an arbitrary string in the same place. On the other hand layout.shapes has a path attribute for type='path' shapes and other attributes for other shapes. Also line.dash is somewhat awkward to describe (a string attribute that nevertheless lists values like an enum?) and shape='<path string>' is a bit funny to read. I don't feel strongly about this though - if others would prefer to add this into the shape attribute that would also be fine.

[archmoj]

I'd like to keep pattern_bars.json as it was before.
Please duplicate the mock and name the new mock zz-pattern_bars-path.json.

This is wonderful feature.
Thanks @alexcjohnson for the PR. 🏆

We may introduce new features like this one in v3.1.0 not v3.0.0; even though one may argue that it's a low risk feature. BTW it requires extra testing on the python side, etc.
cc: @gvwilson