[SEA] Fix order of pre-base reordering Ra and left matras

The code was confused because it was expecting left matra to have
POS_PRE_M, like we do in the Myanmar shaper, but that is not what
we were doing in this shaper.  Rewrite to rely on category only.

Test case: U+AA06,U+AA34,U+AA2F

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

@@ -221,8 +221,9 @@ initial_reordering_consonant_syllable (const hb_ot_shape_plan_t *plan,
       info[i].sea_position() = POS_PRE_C;
       continue;
     }
-    if (info[i].sea_position() < POS_BASE_C) /* Left matra */
+    if (info[i].sea_category() == OT_VPre) /* Left matra */
     {
+      info[i].sea_position() = POS_PRE_M;
       continue;
     }
 

test/shaping/texts/in-tree/shaper-sea/script-cham/misc/misc.txt

@@ -1,2 +1,3 @@
 ꩀꨴ
 ꨗꨪꨇꨮꩃꨯꨗꨱꨧꨩꩂꨯꨨꨱꩃꨨꨮ
+ꨆꨴꨯ