Integerize plater thumbnails
Revert "Remove thumbnail simplification because it caused loss of very thin parts. #1327"

This reverts commit 1210b8989350b96ffbb0ddda39c6132e3eb3f40f.

Conflicts:

	lib/Slic3r/GUI/Plater.pm
Minor error causing support material to fail when first_layer_height was expressed in % (caught by regression test). #1371
Fix wrong slices when many overlapping facets were present in the model
Fix collision detection for brim
New --first-layer-acceleration option
Require --default-acceleration when other acceleration settings are used
Adjust collision detection for brim and include support islands inside the only_retract_when_crossing_perimeters logic
Fix regression causing too much collapsing of infill regions
Adapt avoid_crossing_perimeters code to the recent changes
Offset projection of facets before the final union
Simplify layer change code now that we have distinct support layers
Added some debugging code