Path object improvements

[vzh]

Feature request by Karl Hammar:

Possible not ASAP, but, for paths (^H), to be able to have
round join styles instead of default miter style. See e.g.
http://www.delorie.com/archives/browse.cgi?p=geda-user/2018/07/12/13:25:11

And in the same vein, for filled paths with line width = 0, to really
have zero width, not some default width which will enlargen the
triangle (mostly arrow heads, but filled polygons in general).

I decided to not modify the schematic/symbol file format and do
the latter change in the simple way. Now, when line width of a
path is set to zero, the stroke is not rendered, thus allowing
drawing of filled figures without increasing their size by a half
of the line width, so the object sizes strongly correspond to what
the user wanted.

Attached is a
test symbol
by Karl Hammar.

[graahnul-grom]

An interesting (and very good) side effect: with this patch, different cap styles can be selected for path objects (with width != 0). It used to be always square.

[vzh]

It's not a side effect, but the first one of the requested features. OK, since it works for you, and there is no feedback yet from the original reporter, I'll merge the branch.