We are going to use freetype instead of the old custom TTF parser,
since the old parser has many annoying bugs when handling non-Latin
fonts and fixing it is not really worth the time.
On Windows, Freetype is built from a submodule.
On Linux and OS X, Freetype is provided together with the desktop,
though development files have to be installed separately.
I do not update it anymore and it's probably not the best solution
for distributing SolveSpace on Linux. In the meantime, the builds
from GitHub releases should be convenient enough.
As a side effect, zlib and libpng are now git submodules,
based on their respective official git repositories.
This is necessary, because MinGW has a different ABI and
it cannot use the prebuilt binaries built by MSVC.
The submodules are also used for Windows, for several reasons:
* to allow 64-bit builds;
* to allow using newer MSVC, which doesn't like the prebuilt
libraries;
* to keep the libraries updated.
