Mark Improvements

[johannes-wolf]

Rebased and improved copy of #385.

TODOs:

• Setting flex: false is broken (see tests, wrong mark offset)
• Add open triangle mark
• Add curved triangle mark (barbed)
• Add +, x and * marks
• Allow marks being longer then their path; do not panic if a path is very short
• Fix reversed mark offsetting
• Fix z-axis marks
• Add back “curve snapping” (cubic shorten curve correction)
• Add offset
• Add position (override a mark's position) - Allows relative or absolute mark placement on a path

@fenjalien I've pushed your branch and added some fixes, improvements and missing marks.

I've added linear path extrapolation, which is what TikZ seems to do, too. Marks can “grow” out of the path now.

[johannes-wolf]

I ported 75972d7 over.