Stable spacing

Some bugfixes, picked from the 2.3.57 branch

bugfixes

• disable clearance check if set to 0 #1446
• fix export_to_prusa for object & modifer settings that need the full config as reference #1503
• reprap jerk unit fixed #1500
• fix brim ear generation crash of small surfaces #1513
• Linux & macos fix for coordinate outside allowed range error #66
• fix inside brim going inside raft #1526
• Fix for Linux focus issue #888 by @RealDeuce

note: I haven't tested the linux & macos build.
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