[buffer-diff] Fix check for glyph flag equality

I'm not sure if the old behavior was intentional, but it was checking
that the glyph flags were a subset of the reference buffer's glyph
flags.  I don't see why that is useful.  Fix that.

Then make the buffer-verify code ignore flag differences when verifying
buffers, since our unsafe-to-concat flag at least, is conservative and
not guaranteed to be produced the same in fragments.  See:

https://github.com/harfbuzz/harfbuzz/issues/3888

src/hb-buffer-verify.cc

@@ -186,7 +186,7 @@ buffer_verify_unsafe_to_break (hb_buffer_t  *buffer,
 
   bool ret = true;
   hb_buffer_diff_flags_t diff = hb_buffer_diff (reconstruction, buffer, (hb_codepoint_t) -1, 0);
-  if (diff)
+  if (diff & ~HB_BUFFER_DIFF_FLAG_GLYPH_FLAGS_MISMATCH)
   {
     buffer_verify_error (buffer, font, BUFFER_VERIFY_ERROR "unsafe-to-break test failed.");
     ret = false;
@@ -313,7 +313,6 @@ buffer_verify_unsafe_to_concat (hb_buffer_t        *buffer,
 
   bool ret = true;
   hb_buffer_diff_flags_t diff;
-
   /*
    * Shape the two fragment streams.
    */
@@ -382,7 +381,7 @@ buffer_verify_unsafe_to_concat (hb_buffer_t        *buffer,
    * Diff results.
    */
   diff = hb_buffer_diff (reconstruction, buffer, (hb_codepoint_t) -1, 0);
-  if (diff)
+  if (diff & ~HB_BUFFER_DIFF_FLAG_GLYPH_FLAGS_MISMATCH)
   {
     buffer_verify_error (buffer, font, BUFFER_VERIFY_ERROR "unsafe-to-concat test failed.");
     ret = false;

src/hb-buffer.cc

@@ -2095,7 +2095,7 @@ hb_buffer_diff (hb_buffer_t *buffer,
       result |= HB_BUFFER_DIFF_FLAG_CODEPOINT_MISMATCH;
     if (buf_info->cluster != ref_info->cluster)
       result |= HB_BUFFER_DIFF_FLAG_CLUSTER_MISMATCH;
-    if ((buf_info->mask & ~ref_info->mask & HB_GLYPH_FLAG_DEFINED))
+    if ((buf_info->mask ^ ref_info->mask) & HB_GLYPH_FLAG_DEFINED)
       result |= HB_BUFFER_DIFF_FLAG_GLYPH_FLAGS_MISMATCH;
     if (contains && ref_info->codepoint == dottedcircle_glyph)
       result |= HB_BUFFER_DIFF_FLAG_DOTTED_CIRCLE_PRESENT;