[win32] Remove MemoryBarrier() fallback implementation

I added these because the older mingw32 toolchain didn't have
MemoryBarrier().  The newer mingw-w64 toolchain however has.
As reported by John Emmas this was causing build failure with
MSVC (on glib) because of inline issues.  But that reminded me
that we may be taking this path even if the system implements
MemoryBarrier as a function, which is a waste.  So, just remove
it.
[OTLayout] Rename process() to dispatch()
[OTLayout] Move code around
Add hb_auto_array_t
[ft] Remove TODO item re FT_LOAD_IGNORE_GLOBAL_ADVANCE_WIDTH

That flag is redundant, deprecated, and ignored since April 2011.
From FreeType git log:

commit 8c82ec5b17d0cfc9b0876a2d848acc207a62a25a
Author: Behdad Esfahbod <behdad@behdad.org>
Date:   Thu Apr 21 08:21:37 2011 +0200

    Always ignore global advance.

    This makes FT_LOAD_IGNORE_GLOBAL_ADVANCE_WIDTH redundant,
    deprecated, and ignored.  The new behavior is what every major user
    of FreeType has been requesting.  Global advance is broken in many
    CJK fonts.  Just ignoring it by default makes most sense.

    * src/truetype/ttdriver.c (tt_get_advances),
    src/truetype/ttgload.c (TT_Get_HMetrics, TT_Get_VMetrics,
    tt_get_metrics, compute_glyph_metrics, TT_Load_Glyph),
    src/truetype/ttgload.h: Implement it.

    * docs/CHANGES: Updated.
[ft] Remove TODO items that I'm not going to fix