Generate reversed GSUB sequences for subregion flags too.

Since subregion flag sequences consist of BN and ON they can be
impacted by bidi, and once again we have the problem that these are
processed in visual order so we need GSUB rules such that we can
handle them in either direction.  All subregion flag sequences
contain U+E007F, so we use that as a trigger for adding the
reversed sequence.

We also need to handle emitting the missing flag glyph for the
reversed sequences.

And we also want to strip out tag glyphs when the context is reversed.
This means the chaining context should include 'E007F' as well.

NotoColorEmoji.tmpl.ttx.tmpl

@@ -443,6 +443,7 @@
           </Coverage>
           <BacktrackClassDef Format="1">
             <ClassDef glyph="u1F3F4" class="1"/>
+            <ClassDef glyph="uE007F" class="1"/>
           </BacktrackClassDef>
           <InputClassDef Format="2">
             <ClassDef glyph="uE0030" class="2"/>
@@ -510,6 +511,9 @@
           <LigatureSet glyph="u1F3F4">
             <Ligature components="uE007F" glyph="uFE82B"/>
           </LigatureSet>
+          <LigatureSet glyph="uE007F">
+            <Ligature components="u1F3F4" glyph="uFE82B"/>
+          </LigatureSet>
         </LigatureSubst>
       </Lookup>
     </LookupList>

third_party/color_emoji/add_glyphs.py

@@ -178,7 +178,7 @@ def add_lig_sequence(ligatures, seq, n):
                 print 'lig sequence %s, replace %s with %s' % (
                     tseq, ligatures[tseq], n)
         ligatures[tseq] = n
-        if 'u200D' in seq:
+        if 'u200D' in seq or 'uE007F' in seq:
                 rev_seq = seq[:]
                 rev_seq.reverse()
                 for i in xrange(1, len(rev_seq)):