Better detection of Symbol fonts

> Symbol character sets have a special meaning.
> If the symbol bit (31) is set, and the font file contains a 'cmap'
> subtable for platform of 3 and encoding ID of 1,
> then all of the characters in the Unicode range 0xF000 - 0xF0FF
> (inclusive) will be used to enumerate the symbol character set.

If we detected the font has a symbol character set, report no other
writing systems support.

Change-Id: I1030f3339c166ffd03c7caee1b1b26010dfdc314
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

src/gui/text/qplatformfontdatabase.cpp

@@ -525,7 +525,8 @@ enum {
     ThaiCsbBit = 16,
     JapaneseCsbBit = 17,
     KoreanCsbBit = 19,
-    KoreanJohabCsbBit = 21
+    KoreanJohabCsbBit = 21,
+    SymbolCsbBit = 31
 };
 
 /*!
@@ -610,6 +611,11 @@ QSupportedWritingSystems QPlatformFontDatabase::writingSystemsFromTrueTypeBits(q
         hasScript = true;
         //qDebug("font %s supports Korean", familyName.latin1());
     }
+    if (codePageRange[0] & (1 << SymbolCsbBit)) {
+        writingSystems = QSupportedWritingSystems();
+        hasScript = false;
+    }
+
     if (!hasScript)
         writingSystems.setSupported(QFontDatabase::Symbol);