Refactor Thumbrenderer

[Thesacraft]

I refactored Thumbrenderer to remove duplicate code.
I merged the render and render_big methods into one _render method
I'm open to any suggestions and changes

[CyanVoxel]

Overall this looks pretty good to me! The thumb renderer is about to be sort of a hot spot in the code with lots of activity (#171, #175, #179, #183, raw-image-previews, etc.), so while I'm open to an even larger refactor things will probably need to be done in smaller chunks over time after this PR. Consolidating the render and render_big methods was the huge thing that needed done though, so I'm happy to have that improved in time for the next round of fixes and additions!

With all that being said, I'll be pulling #156 before this which refactors out all uses of os.path in favor of the pathlib library going forward. I'll also be pulling the MyPy workflow from #169 which will also shake things up here. I apologize for the additional refactoring/formatting.

[Thesacraft]

Thanks for resolving the conflicts. i was just waiting for the os.paths pr to get merged and do it all in one big cleanup, haha. But i really appreciate the help.