Don't ever draw lines to explicit M/m commands.

#1365 converted multiple M/m coordinates to L/l commands when importing paths. The `_render` function was already attempting to connect those coordinates, but that's no longer necessary as the only consecutive M/m commands in `_render` were explicitly defined as M/m commands.
Add spec for multiple M/m commands being preserved.

This wasn't ever broken, but taken with the previous spec it makes more clear that there's a difference between `M 1,2 3,4` and `M 1,2 M 3,4`.
Merge pull request #1380 from rodovich/explicit-moveto

Don't draw lines for consecutive explicit "moveto" commands