[beta] Thick first layer edition

Merged against prusaslicer 2.2.0, see their post to see the changelog.

known issue

• be sure to use a negative number in the first layer XY compensation (not the default value in prusa setting). Good profiles has been put int he 2.2.48.1 release.

new & changes

• added a "first layer flow" setting to be able to over-extrude on the first layer. I didn't want to add it but my unreliable bed height left me no choice. Please thicken the first layer width before messing with that boy.
• "external perimeter first" can now be activated only for the external or internal side (holes) if needed.
• add previous_extruder, next_extruder, layer_num and layer_z as variable available into start/end filament gcode macros.

bugfixes

• some fixes for macos compile & launch.
• "seam in vase mode" option fixes. It can now be activated and it's as bad as it used to be
• fix ironing for support interface pattern #153
• fix "overlapping perimeters" activation condition
• improve "smoothing curves" a bit (remove co-linear points more aggressively)
• fix a gapfill edge case: higher max width allowed

macos and linux build will be available here.