[use] Allow up to two medial-below letters

Fixes https://github.com/behdad/harfbuzz/issues/376

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

@@ -89,7 +89,8 @@ SMBlw	= 42; # SYM_MOD_BELOW
 
 
 consonant_modifiers = CMAbv* CMBlw* ((H B | SUB) VS? CMAbv? CMBlw*)*;
-medial_consonants = MPre? MAbv? MBlw? MPst?;
+# Override: Allow two MBlw. https://github.com/behdad/harfbuzz/issues/376
+medial_consonants = MPre? MAbv? MBlw?.MBlw? MPst?;
 dependent_vowels = VPre* VAbv* VBlw* VPst*;
 vowel_modifiers = VMPre* VMAbv* VMBlw* VMPst*;
 final_consonants = FAbv* FBlw* FPst* FM?;

test/shaping/tests/use-syllable.tests

 fonts/sha1sum/96490dd2ff81233b335a650e7eb660e0e7b2eeea.ttf::U+AA00,U+AA2D,U+AA29:[a_cham=0+1121|uSign_cham=0@14,0+0|.notdef=0+600]
+fonts/sha1sum/e68a88939e0f06e34d2bc911f09b70890289c8fd.ttf::U+AA00,U+AA34,U+AA36:[raMedial_cham_pre=0+400|a_cham=0+1121|waMedial_cham=0@-32,0+0]
+fonts/sha1sum/e68a88939e0f06e34d2bc911f09b70890289c8fd.ttf::U+AA00,U+AA35,U+AA33:[a_cham=0+1121|laMedial_cham=0@-32,0+0|yaMedial_cham=0+542]
+fonts/sha1sum/e68a88939e0f06e34d2bc911f09b70890289c8fd.ttf::U+AA00,U+AA35,U+AA36:[a_cham=0+1121|laMedial_waMedial_cham=0@43,0+0]