Makes debug output much more pleasant.

More to come, for APPLY, CLOSURE, etc.

We have varargs in the trace interface now.  To be used soon...

Users should #include <hb.h> (or hb-ft.h, hb-glib.h, etc), but
never things like hb-shape.h directly.  This makes it easier to
refactor headers later on without breaking compatibility.

Fixes build with MSVC.

We need this to set data on objects safely without worrying that some
other thread unsets it by setting it at the same time.

Where causing issues for people with MSVC.

Not complete yet, font selection doesn't work.  But hey it shapes!

This is not supposed to be a production backend, more like a testing
backend.

With these changes, on Ubuntu I can do:

./configure --host=i586-mingw32msvc  && make

Number of failing shape-complex tests goes from 125 down to 94.

Next: Add Ra handling and it's fair to say we kinda support Indic :).

Looks *so* much nicer!

Buffer debugging coming soon.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=644184
among others.

Shapers now can request segmented feature application by calling
add_gsub_pause() or add_gpos_pause().  They can also provide a
callback to be called at the pause.  Currently the Arabic shaper
uses pauses to enforce certain feature application.  The Indic
shaper can use the same facility to pause and do reordering in the
callback.

The test/object test is passing again, instead of deadlocking.