Let hb_shape() guess script and direction...

- 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.

src/hb-ot-shape.cc

diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index 92c3925..e6b3f3e 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -64,6 +64,7 @@
       break;
     case HB_DIRECTION_TTB:
     case HB_DIRECTION_BTT:
+    case HB_DIRECTION_INVALID:
     default:
       break;
   }
@@ -170,7 +171,7 @@
 
   /* TODO vertical */
   if (HB_DIRECTION_IS_HORIZONTAL (direction) &&
-      direction != _hb_script_get_horizontal_direction (c->buffer->props.script))
+      direction != hb_script_get_horizontal_direction (c->buffer->props.script))
   {
     hb_buffer_reverse_clusters (c->buffer);
     c->buffer->props.direction = HB_DIRECTION_REVERSE (c->buffer->props.direction);