Only do fallback Hebrew composition if no GPOS 'mark' available

Apparently some modern fonts have proper GPOS mark positioning
tables, but undesirable precomposed forms!  See thread
"Hebrew composition to presentation forms" and:

  http://tex.stackexchange.com/questions/156775/having-trouble-with-vowel-positioning-in-ezra-sil-xelatex

Test case: U+fb1d,05d9,05b4

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

diff --git a/src/hb-ot-shape-complex-hebrew.cc b/src/hb-ot-shape-complex-hebrew.cc
index efef8c1..6bb6043 100644
--- a/src/hb-ot-shape-complex-hebrew.cc
+++ b/src/hb-ot-shape-complex-hebrew.cc
@@ -70,7 +70,7 @@
 
   bool found = c->unicode->compose (a, b, ab);
 
-  if (!found)
+  if (!found && !c->plan->has_mark)
   {
       /* Special-case Hebrew presentation forms that are excluded from
        * standard normalization, but wanted for old fonts. */