feat(ui): implement STL preview renderer (#1429)

* feat: imlpement custom STL preview renderer from scratch

* perf: optimize STL renderer by circumventing expensive PIL draw calls

* perf: optimize loading of ascii-formatted STL files

* style: format stl_renderer.py with ruff

* feat(STL renderer): fix visual artifacts + cleanup

* feat: move stl rendering code into the previews/renderers/stl.py

* chore: retrigger CI

* refactor: remove unnecessary color conversion

* refactor: apply suggestion from @CyanVoxel

* fix: remove unnecessary QPixMap locking

Author: ludvig-sandh