usp10: In Arabic only fallback to presentation form B for isolated glyphs.

dlls/usp10/shape.c

@@ -1567,6 +1567,8 @@ static void ContextualShape_Arabic(HDC hdc, ScriptCache *psc, SCRIPT_ANALYSIS *p
 
         if (!shaped)
         {
+            if (context_shape[i] == Xn)
+            {
                 WORD newGlyph = pwOutGlyphs[i];
                 if (pwcChars[i] >= FIRST_ARABIC_CHAR && pwcChars[i] <= LAST_ARABIC_CHAR)
                 {
@@ -1575,6 +1577,7 @@ static void ContextualShape_Arabic(HDC hdc, ScriptCache *psc, SCRIPT_ANALYSIS *p
                     if (context_char != pwcChars[i] && GetGlyphIndicesW(hdc, &context_char, 1, &newGlyph, 0) != GDI_ERROR && newGlyph != 0x0000)
                         pwOutGlyphs[i] = newGlyph;
                 }
+            }
             i++;
         }
     }