[Indic] Accept a forced Rakar sequence at the end of syllable

In Sinhala, Rakar is formed by Al-Lakuna,ZWJ,Ra.  If you put that at the
end of a Consonant,Matra syllable, you get a dotted-circle from
Uniscribe.  Apparently adding a ZWJ before the Al-Lakuna "fixes" that.
And people have been encoding that sequence...  So, allow a forced
"ZWJ,Virama,ZWJ,Ra" sequence at the of syllables.

Fixes some 100 or more of Sinhala failures.  Now at 622 only (0.23%).

src/hb-ot-shape-complex-indic-machine.rl

@@ -63,7 +63,8 @@ h = H | Coeng;			# is_halant_or_coeng
 reph = (Ra H | Repha);		# possible reph
 
 cn = c.n?;
-matra_group = z*.M.N?.H?;
+forced_rakar = ZWJ H ZWJ Ra;
+matra_group = z*.M.N?.(H | forced_rakar)?;
 syllable_tail = SM? (Coeng (cn|V))? (VD VD?)?;
 place_holder = NBSP | DOTTEDCIRCLE;
 halant_group = (z?.h.z?);

test/shaping/texts/in-tree/shaper-indic/indic/script-sinhala/misc/misc.txt

@@ -5,3 +5,5 @@
 කෝ
 කෝ
 ස්ට්‍රේ
+ක‍්‍රම
+ශී‍්‍ර