Implement more granular cluster-merging

TODO: Documentation.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=71445

src/hb-ot-shape-complex-hangul.cc

diff --git a/src/hb-ot-shape-complex-hangul.cc b/src/hb-ot-shape-complex-hangul.cc
index 6ac18b0..763dbf0 100644
--- a/src/hb-ot-shape-complex-hangul.cc
+++ b/src/hb-ot-shape-complex-hangul.cc
@@ -209,13 +209,8 @@
 	  hb_glyph_info_t tone = info[end];
 	  memmove (&info[start + 1], &info[start], (end - start) * sizeof (hb_glyph_info_t));
 	  info[start] = tone;
+	  buffer->merge_out_clusters (start, end + 1);
 	}
-	/* Merge clusters across the (possibly reordered) syllable+tone.
-	 * We want to merge even in the zero-width tone mark case here,
-	 * so that clustering behavior isn't dependent on how the tone mark
-	 * is handled by the font.
-	 */
-	buffer->merge_out_clusters (start, end + 1);
       }
       else
       {
@@ -296,7 +291,8 @@
 	}
 	else
 	  end = start + 2;
-	buffer->merge_out_clusters (start, end);
+	if (buffer->cluster_level == HB_BUFFER_CLUSTER_LEVEL_MONOTONE_GRAPHEMES)
+	  buffer->merge_out_clusters (start, end);
 	continue;
       }
     }
@@ -368,7 +364,8 @@
 	  info[i++].hangul_shaping_feature() = VJMO;
 	  if (i < end)
 	    info[i++].hangul_shaping_feature() = TJMO;
-	  buffer->merge_out_clusters (start, end);
+	  if (buffer->cluster_level == HB_BUFFER_CLUSTER_LEVEL_MONOTONE_GRAPHEMES)
+	    buffer->merge_out_clusters (start, end);
 	  continue;
 	}
       }