All these changes will allow us to more easily add Lol Engine as a
submodule of another Git project.

There might be issues with lolremez but at first sight it seems to work.

For now, all it can do is read and decompress .p8.png files.

Add mingw-std-threads project to external/ for full C++11 thread support, fix
an incorrect Win32 fix I did in PEGTL, work around mingw’s <io.h> declaring a
conflicting eof symbol, and fix a capitalisation error in log.cpp.

The containers no longer force the user to use the ptrdiff_t type for
size information. For convenience, size() now always returns an int,
and the size_s() method offers a way to handle arrays with more than
2 billion elements. Internally, we still use ptrdiff_t, though.

Since so much code had to be changed, I took the opportunity to get
rid of capitalised accessors for classes that are lowercase.

support for enable_if and other template metaprogramming techniques),
for Clang (related to static const template class member initialisation)
and for GCC (related to forward declaration of non-int enum classes).

point parsing, add support for unary plus and minus.

and 2) fuck the Visual Studio compiler that crashes when confused.

MASK to SWIZZLE in the vector templates, rename matrix<> to mat<> for
consistency, implement transposition for all matrix sizes, make matrix
columns private and only accessible through operator[].