* it replace rectilinear fill by gapfill when width < spacing * 1.6
* it permit to fill spaces too thin to be filled by infill (if it's not already filled by the perimeter gapfill)
* it print the gapfill after the rectilinear infill.
* the exact volume option only do his thing on the rectilinear area, not the gapfill one (as it's already done in the medial axis).
* to build the tests on windows, you need vs2015 or 2017 with "CRT SDK"
so you have to also build all deps & slic3r against this compiler.
* I didn't tried to build the tests on linux/osx, maybe some cmkae changes are needed.
* I added a slic3r_lib project to be able to link statically the test against all the slic3r project.
* tests will need many changes to be able to use the new codebase. only the ones in the cmakelists are done right now.
* some test may be in error, because the behavior of the test vs slic3r isn't defined well enough to make a decision of which one is wrong.