[Indic] Tamil does not have half-forms either

The Win7 Tamil font does not realy on this behavior, but the WinXP
version does.  Handle Tamil like Malayalam: Matras always move to
before base.

WinXP Tamil failures went down from 168964 (15.4752%) to 167
(0.0152953%) (two orders of magnitude reduction!).

Included in this is a minor fixup that actually fixed a few tests
with non-Tamil too.  Numbers at:

BENGALI: 353997 out of 354285 tests passed. 288 failed (0.0812905%)
DEVANAGARI: 707339 out of 707394 tests passed. 55 failed (0.00777502%)
GUJARATI: 366489 out of 366506 tests passed. 17 failed (0.0046384%)
GURMUKHI: 60769 out of 60809 tests passed. 40 failed (0.0657797%)
KANNADA: 951086 out of 951913 tests passed. 827 failed (0.0868777%)
KHMER: 299106 out of 299124 tests passed. 18 failed (0.00601757%)
LAO: 53611 out of 53644 tests passed. 33 failed (0.0615167%)
MALAYALAM: 1048104 out of 1048416 tests passed. 312 failed (0.0297592%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271747 out of 271847 tests passed. 100 failed (0.0367854%)
TAMIL: 1091837 out of 1091837 tests passed. 0 failed (0%)
TELUGU: 970558 out of 970573 tests passed. 15 failed (0.00154548%)
TIBETAN: 208469 out of 208469 tests passed. 0 failed (0%)

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

@@ -917,11 +917,11 @@ final_reordering_syllable (const hb_ot_shape_plan_t *plan,
     /* If we lost track of base, alas, position before last thingy. */
     unsigned int new_pos = base == end ? base - 2 : base - 1;
 
-    /* Malayalam does not have "half" forms or explicit virama forms.
-     * The glyphs formed by 'half' are Chillus.  We want to position
-     * matra after them all.
+    /* Malayalam / Tamil do not have "half" forms or explicit virama forms.
+     * The glyphs formed by 'half' are Chillus or ligated explicit viramas.
+     * We want to position matra after them.
      */
-    if (buffer->props.script != HB_SCRIPT_MALAYALAM)
+    if (buffer->props.script != HB_SCRIPT_MALAYALAM && buffer->props.script != HB_SCRIPT_TAMIL)
     {
       while (new_pos > start &&
 	     !(is_one_of (info[new_pos], (FLAG (OT_M) | FLAG (OT_H) | FLAG (OT_Coeng)))))
@@ -941,7 +941,7 @@ final_reordering_syllable (const hb_ot_shape_plan_t *plan,
         new_pos = start; /* No move. */
     }
 
-    if (start < new_pos)
+    if (start < new_pos && info[new_pos].indic_position () != POS_PRE_M)
     {
       /* Now go see if there's actually any matras... */
       for (unsigned int i = new_pos; i > start; i--)