[OT] Insert dotted-circle for run-initial marks

Unfortunately if the font has GPOS and 'mark' feature does
not position mark on dotted-circle, our inserted dotted-circle
will not get the mark repositioned to itself.  Uniscribe cheats
here.

If there is no GPOS however, the fallback positioning kicks in
and sorts this out.

I'm not willing to address the first case.

src/hb-ot-shape.cc

@@ -234,6 +234,37 @@ hb_set_unicode_props (hb_buffer_t *buffer)
     _hb_glyph_info_set_unicode_props (&buffer->info[i], buffer->unicode);
 }
 
+static void
+hb_insert_dotted_circle (hb_buffer_t *buffer, hb_font_t *font)
+{
+  /* TODO One day, when we keep _before_ text for the buffer, take
+   * that into consideration.  For now, insert dotted-circle if the
+   * very first character is a non-spacing mark. */
+  if (_hb_glyph_info_get_general_category (&buffer->info[0]) !=
+      HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK)
+    return;
+
+  hb_codepoint_t dottedcircle_glyph;
+  if (!font->get_glyph (0x25CC, 0, &dottedcircle_glyph))
+    return;
+
+  hb_glyph_info_t dottedcircle;
+  dottedcircle.codepoint = 0x25CC;
+  _hb_glyph_info_set_unicode_props (&dottedcircle, buffer->unicode);
+
+  buffer->clear_output ();
+
+  buffer->idx = 0;
+  hb_glyph_info_t info = dottedcircle;
+  info.cluster = buffer->cur().cluster;
+  info.mask = buffer->cur().mask;
+  buffer->output_info (info);
+  while (buffer->idx < buffer->len)
+    buffer->next_glyph ();
+
+  buffer->swap_buffers ();
+}
+
 static void
 hb_form_clusters (hb_buffer_t *buffer)
 {
@@ -526,6 +557,7 @@ hb_ot_shape_internal (hb_ot_shape_context_t *c)
   c->buffer->clear_output ();
 
   hb_set_unicode_props (c->buffer);
+  hb_insert_dotted_circle (c->buffer, c->font);
   hb_form_clusters (c->buffer);
 
   hb_ensure_native_direction (c->buffer);