fixed a bug with checkPolygons

It took me quite some time to debug this.
Basically shape2 & shape 1 were tested against
shape 1 but since those are polygons, depending
on who's first in the argument, the unitVector
won't be the most relevant to resolve the
collision.

shape2 & shape1 also need to be tested against
shape2.

I started changing code in checkPolygons itself
but googling a bit i found the source of your
port and obviously calling checkPolygons twice
is the most elegant way here. You still need a
way to give the order in which you're testing.
The port is using a flip boolean as parameter
so i basically ported back that piece of code
again.

So now for consistency i have to edit my
checkCircleVsPolygon/checkPolygonVsCircle combo
and will add a flip parameter as i did with
checkPolygons. WILL DO in next commits.

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