I'm going to give it a shot to use this for bidi text support, it looks
like it's a pretty lightweight, compatible and low-dependency library
which is definitely a plus. We'll still need to do reshaping ourselves,
but that's the easy part compared to bidi.
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.
