png: Replaced libpng with lodepng.

This was explicitly done to remove code that was possibly still licensed
under the LGPL in our own libpng glue code, but also lodepng is significantly
less bulky that libpng without any apparent loss in functionality. Now it's
just a single C file that is #included from IMG_png.c, and unless SDL_image
is built without PNG support at all, is always available at runtime. The
shared libraries, the big source trees: gone. zlib and miniz are also removed,
as they were only there to support libpng. lodepng provides its own internal
compression code.

Saving and loading are both supported, as are paletted images, etc. This has
been run through an initial test suite of PNG images, but needs more complete
testing (and likely minor updates to some of the project files).