Fix some bugs in bridge detection

There is problem with different angle convention used for infill and for rest of geometry. 
Geometry uses zero pointing east and angle growing counterclockwise, infill code uses zero toward north (infill with angle zero is vertical) and angle increasing clockwise (passed angle is used to rotate fill boundaries).
Angles are translated here, but this fix should be only temporary.

In my opinion changing infill code (rotate by -angle, use vertical infill pattern) is only reasonable long-term solution. Also angle parameters to slic3r should be converted to radians as soon as possible and used consistently.