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
Merge branch 'master' into adaptive-slicing