Initial relative moveto incorrectly merged

[jzbrooks]

The particular problem in #101 was incidentally addressed in #109 because the paths happen to overlap, but there are two cases that probably deserve consideration when merging paths that don't overlap.

1. If the initial moveto is relative and it has no implicit parameters, it should be made absolute before merging. Initial movetos are always relative to the origin (e.g. absolute), but if it is merged into a new path, then semantically it would be relative to the current point in the path, which wouldn't be correct.
2. If the initial moveto is relative and it has implicit lineto parameters (coordinate pairs after the first), then the command and all coordinates will need to be absolutized.