[ot-layout] Add central baselines

Add HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_CENTRAL
and HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_CENTRAL
which are the centers of the ideographic em-box
and face box.

src/hb-ot-layout.cc

@@ -2241,6 +2241,46 @@ hb_ot_layout_get_baseline_with_fallback (hb_font_t                   *font,
       *coord = font->x_scale * 6 / 10; // FIXME makes assumptions about origin
     break;
 
+  case HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_CENTRAL:
+    {
+      hb_position_t top, bottom;
+      hb_ot_layout_get_baseline_with_fallback (font,
+                                               HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_TOP_OR_RIGHT,
+                                               direction,
+                                               script_tag,
+                                               language_tag,
+                                               &top);
+      hb_ot_layout_get_baseline_with_fallback (font,
+                                               HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_BOTTOM_OR_LEFT,
+                                               direction,
+                                               script_tag,
+                                               language_tag,
+                                               &bottom);
+      *coord = (top + bottom) / 2;
+
+    }
+    break;
+
+  case HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_CENTRAL:
+    {
+      hb_position_t top, bottom;
+      hb_ot_layout_get_baseline_with_fallback (font,
+                                               HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_TOP_OR_RIGHT,
+                                               direction,
+                                               script_tag,
+                                               language_tag,
+                                               &top);
+      hb_ot_layout_get_baseline_with_fallback (font,
+                                               HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_BOTTOM_OR_LEFT,
+                                               direction,
+                                               script_tag,
+                                               language_tag,
+                                               &bottom);
+      *coord = (top + bottom) / 2;
+
+    }
+    break;
+
   case _HB_OT_LAYOUT_BASELINE_TAG_MAX_VALUE:
   default:
     *coord = 0;

src/hb-ot-layout.h

@@ -487,9 +487,11 @@ hb_ot_layout_feature_get_characters (hb_face_t      *face,
  * if the direction is horizontal or vertical, respectively.
  * @HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_TOP_OR_RIGHT: Ideographic character face top or right edge,
  * if the direction is horizontal or vertical, respectively.
+ * @HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_CENTRAL: The center of the ideographic character face. Since: REPLACEME
  * @HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_BOTTOM_OR_LEFT: Ideographic em-box bottom or left edge,
  * if the direction is horizontal or vertical, respectively.
  * @HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_TOP_OR_RIGHT: Ideographic em-box top or right edge baseline,
+ * @HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_CENTRAL: The center of the ideographic em-box. Since: REPLACEME
  * if the direction is horizontal or vertical, respectively.
  * @HB_OT_LAYOUT_BASELINE_TAG_MATH: The baseline about which mathematical characters are centered.
  * In vertical writing mode when mathematical characters rotated 90 degrees clockwise, are centered.
@@ -503,8 +505,10 @@ typedef enum {
   HB_OT_LAYOUT_BASELINE_TAG_HANGING			= HB_TAG ('h','a','n','g'),
   HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_BOTTOM_OR_LEFT	= HB_TAG ('i','c','f','b'),
   HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_TOP_OR_RIGHT	= HB_TAG ('i','c','f','t'),
+  HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_CENTRAL		= HB_TAG ('I','c','f','c'),
   HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_BOTTOM_OR_LEFT	= HB_TAG ('i','d','e','o'),
   HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_TOP_OR_RIGHT	= HB_TAG ('i','d','t','p'),
+  HB_OT_LAYOUT_BASELINE_TAG_IDEO_EMBOX_CENTRAL		= HB_TAG ('I','d','c','e'),
   HB_OT_LAYOUT_BASELINE_TAG_MATH			= HB_TAG ('m','a','t','h'),
 
   /*< private >*/