Spacing_Mark is the current Unicode long-name for this property value.
The previous name was wrongly carried from glib.

Useful in C API only.

It uses locale information to detect default language.  It's used by
hb_shape() whenever language is not set on the buffer.

Not sure how to properly test it in the test suite.  Tested by observing
that with DejaVu Sans we select the proper local glyph version for U+431
under Serbian locale.  See http://www.pango.org/ScriptGallery

Update to http://unicode.org/iso15924

Fixes some of the test failures in test-unicode with ICU.  Still
one more to fix before the test passes.

And hb-view --direction argument.

This simplifies the code, reduces static data, and makes the design
more extensible to future additions of new scripts.

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.

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.

- Rename HB_SCRIPT_INVALID_CODE to HB_SCRIPT_INVALID

- Add HB_DIRECTION_INVALID

- Make hb_script_get_horizontal_direction() public

- Make hb_shape() guess script from buffer text (first non-common
  non-inherit script) if buffer script is set to HB_SCRIPT_INVALID (this
  is NOT the default.)

- Make hb_shape() guess direction from buffer script if buffer direction
is set to HB_DIRECTION_INVALID (this is NOT the default.)

- Make hb-view.c set INVALID script and direction on the buffer.

The above changes are meant to make hb-view fairly useful for uni-script
uni-direction text.  The guessing behavior however is NOT the default of
hb_shape() and must be asked for explicitly.  This is intended, because
the guess is not a suitable substitute to full-fledged bidi and script
segmentation.  It's just a testing tool.

hb_font_set_scale() now sets the value to be used to represent a unit
pixel.  For example, if rendering a 10px font with a 26.6 representation,
you would set scale to (10 << 6).  For 10px in 16.16 you would set it to
(10 << 16).  This space should be the same space that the get_glyph_metrics
and get_kerning callbacks work in.

We already depend on the exact values of the direction enum
in HB_DIRECTION_REVERSE(), so we may as well use that.

In anticipation for buffer revamp coming.

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.