more fixes for #1002 - process_bridges() and ExPolygon->safety_offset()

Isolated clockwise-wound hole polygons should be converted to ccw
and the offset value negated before running them through a Clipper
offset. Results can then be converted back to cw. Otherwise hole
windings get reversed.
Also extended previous process_bridges() fix a bit further - accounting
for holes in bridges or overhangs a bit earlier.