2.8.2 pgce

Update libpng

Statically links in zlib now, so explicit zlib DLL can be removed.

2.8.0 pgce

A release build bundle of SDL_image 2.8.0 using libpng and libjpeg. The default SDL_image builds use stbimage, which is smaller, but less performant and not fully conformant with behavior we've put into test cases.

(This is built against dfee9e2 despite the tag of this release)