Suppress some compiler warnings, fix few possible bugs.

[buzzhuzz]

Description

Suppress compiler warnings we not going to fix. Fix few obvious errors.

While looking at issue #6137 I found suspicious code:

https://github.com/SoftFever/OrcaSlicer/blob/5e4dd4d84792682accff3ae8c965444fcb5d242e/src/libslic3r/Extruder.cpp#L12

This led me to an idea that no one is looking at warnings produced by compilers. Currently there are 3056 warnings emitted for OrcaSlicer build using GCC, nearly half of which are unused functions and variables. All that noise distracts developer making them completely ignore compiler output and because of this some obvious mistakes makes into the code base.

[buzzhuzz]

Another candidate for suppression is -Wsign-compare which produces 1145 warnings.