Improve and fix behaviour when collapsing straight paths segments

Created by: Ede123

• Do not collapse straight path segments in paths that have intermediate markers (see #145 (closed)). The intermediate nodes might be unnecessary for the shape of the path, but their markers would be lost.
• Collapse subpaths of moveto m and lineto l commands if they have the same direction (before we only collapsed horizontal/vertical h/v lineto commands)
• Attempt to collapse lineto l commands into a preceding moveto m command (these are then called "implicit lineto commands")
• Preserve empty path segments if they have stroke-linecap set to round or square. They render no visible line but a tiny dot or square.