Addressing Nikolay Sivov reviews on harfbuzz mailing list

No longer noticeable memory increase on create/destroy iterations,
highly better than current state of uniscribe backend

This reduces memory consumption of my iterated font create/destroy cycle test
significantly and makes it much better than uniscribe backend even

With help of
https://dxr.mozilla.org/mozilla-central/source/gfx/2d/NativeFontResourceDWrite.cpp

Actually copyedited same logic from Uniscribe to make it just work

To lower the number of gtk-doc warnings.