I really shall find a way to automate the test enumeration.  Otherwise
it's too easy to add a test and wrongly hook it up.  Did it twice today.

It's just sugar.

The problem with HB_TAG_STR() was that it expected a string of size 4
exactly, and unlike hb_tag_from_string() it doesn't pad the tag with
space characters.  So, the new name is more appropriate.

Uses the glib testing framework.  Wrote unit tests for hb-common.h
types.

Never hurts.

Since we now assert thos in hb-private.h, the int types cannot be wrong.
(Except for when someone else includes hb-common.h in a very broken
configuration, but that's not our problem!)

Plus, we don't use inline in the public headers, so remove that too.

That better matches OpenType spec.  Note that we enable it for all
Arabic-shaper scripts.  Ie. we enable it by default for Syriac too,
but the SyriacOT spec does not require it.  I think this is a more
useful compromise than special-casing for Arabic script alone.

For now we don't use anything from the locale, but we should default
to using $LANG, etc, if --language is not specified.  Right?

The --features parsing handles errors now.  More importantly, it
allos limiting individual features to specific byte ranges.  The
format is Python-esque.  Here is how it all works:

  Syntax:	Value:	Start:	End:

Setting value:
  "kern"	1	0	∞	# Turn feature on
  "+kern"	1	0	∞	# Turn feature off
  "-kern"	0	0	∞	# Turn feature off
  "kern=0"	0	0	∞	# Turn feature off
  "kern=1"	1	0	∞	# Turn feature on
  "kern=2"	2	0	∞	# Choose 2nd alternate

Setting index:
  "kern[]"	1	0	∞	# Turn feature on
  "kern[:]"	1	0	∞	# Turn feature on
  "kern[5:]"	1	5	∞	# Turn feature on, partial
  "kern[:5]"	1	0	5	# Turn feature on, partial
  "kern[3:5]"	1	3	5	# Turn feature on, range
  "kern[3]"	1	3	3+1	# Turn feature on, single char

Mixing it all:

  "kern[3:5]=0"	1	3	5	# Turn feature off for range

Avoids recreating the face the second time we call draw().

Frees all allocated memory before exiting.  Useful for valgrind run.

Accepts values like:

  --features="-mkmk,aalt=2,calt=1,+cswh"

A minus sign means "=0", a plus sign means "=1".  Default is "=1".
A minus sign overrides an explicit value.

Also adds --script support to hb-view.

If a script tag is not known to us, we pass the ISO 15924 tag around.
Right now, the OT layer ignores that, but we can fix it to blindly
convert that to an OT script tag.