As a part of meson migration, this builds hb-shape dependencies
using meson subprojects and removes different unrelated mingw
files we've put on source root about things that aren't really our
concern. This also simplifies all the instructions we've put in
README.mingw.md and makes it easy to build the project on
distros that no mingw libraries (such i686-win32-{freetype,glib,...}
are packaged. The known catch is however lack of hb-view as its need
for build of cairo which apparently isn't that straightforward.
