Make path helpers work correctly with decimal values

[cbranch101]

Fixes FormidableLabs/victory#753

Using math.round in all of these functions meant that the round could go one way or the other if provided decimal x or y values, which in turn would mess up the point shapes.

Everywhere I could, just swapped functions over to use Math.floor. Unfortunately, for plus, square, and diamond that lead to dramatically different results. For those, I had to rework the way the shapes were drawn quite a bit. For the most part, the shapes should be very similar. Using floor instead of round meant that some of the shapes decreased in size ever so slightly, and I had to completely rethink how plus was drawn, so the version I have is more symmetrical than the old version, but it seemed worth it to get rid of the old shapes.

For comparison's sake, here's all of the shapes, going from size 2 to 13 in increments of three. The right icon in each pair is the old implementation of path-helpers and left icon is my implementation.

[boygirl]

@cbranch101 Thank you for this fix!