Update SDL_image to 2.8.8

[StephenCWills]

This resolves #7894

As the issue suggests, the newer version of SDL_image uses stb_image for PNG, but only for loading. We still need libpng for screenshot functionality. But the dependency is pulled in automatically thanks to the native CMake support.

[StephenCWills]

For Xbox One, we need to modify devilutionx.vcxproj to reference SDL2_image-static.lib instead.

I'm not entirely sure what's going on with iOS. Seems like maybe SDL_image assumes we aren't building in "automatic reference counting mode"? Even so, there seems to be a straight-up casting error.

Xbox maybe doesn't have a compatible SDL version.

Linux x64 Tests is complaining about PIC. I'm not sure why, as it seems to me that PIC is on by default. Also, the errors are in SDL2.a?
https://github.com/libsdl-org/SDL_image/blob/feffbe13d90a893d00d0b099f63fa59e8f64ce66/CMakeLists.txt#L59

The rest of the errors appear to be sourceware.org issues.

I'm not certain if any of these errors are showstoppers at this point, but iOS and Xbox seem like they'll be difficult to deal with.