This means we are no longer copy-pasting PhysFS source files directly.
Since the source files reside in a src/ subdirectory, the paths in the
CMakeLists.txt have to be adjusted.
We are no longer copy-pasting LodePNG source files directly.
As we can't rename lodepng.cpp to lodepng.c in the submodule itself, we
need to make a wrapper file, lodepng_wrapper.c, that #includes
lodepng.cpp, but gets compiled as C.