Case-insensitive ZIP loading

[NotHyper-474]

This mod shows an interesting problem that is, the modder might not care enough for the letter case of the files, which may cause inconsistent behavior not only in case-sensitive platforms (like Linux) but also when trying to load their mod as a ZIP.

This PR aims to solve at least the ZIP part. I might look into the Linux issue later.

[EliteMasterEric]

This code seems good but I wonder if the behavior could be made optional (via a compilation flag or similar) since some applications may expect case-sensitive asset loading. Or maybe we could do the opposite and ensure case-sensitive asset loading on Windows.

[NotHyper-474]

Done. Now you need to set a POLYMOD_ZIP_INSENSITIVE define to enable it, (although maybe it should be enabled by default?) also it needs #208 to work properly.

Or maybe we could do the opposite and ensure case-sensitive asset loading on Windows.

Definitely would break a mod or two (I know for sure QT: Rewired would be among these as it has a couple of vocals that don't load on Linux). Personally I'm more inclined towards ensuring case-insensitive loading on Linux instead, but up to you.

[NotHyper-474]

No idea why but hxcpp is now imploding with this code

Edit: Fixed it, turns out both the getter and the variable needed to have the same exact type...