Make Slic3r::Point coordinates doubles

This branch converts the Slic3r::Point x and y coordinates to `double`s (first commit)

By doing so, we avoid most of the rounding issues incurred from going back and forth between longs and doubles on 32 bit archs.

Only one unit test in `t/polyclip.t` had to be adapted (second commit), which lets me think that the changes are pretty sound.

AFAICT, Boost::Geometry::Utils happily does arithmetic on any kind of values passed to it (therefore doubles are ok), so we only need to take care when interfacing with Clipper, which only works on integer values. To do so, we properly round the values before passing them on to Clipper (third commit).

Closes: #1486, #1464, #1445