Treat color (ARGB) glyphs, e.g. Emoji, as having unreliable glyph outlines

This is used by the scene graph to automatically switch over from distance
field text to native text rendering for the given glyph node, which allows
mixing regular text with Emoji in e.g. a Text item without having to set
renderType to Text.NativeRendering.

Change-Id: I5d96d1dab329a975e3442284bf4c5a82174177c9
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

src/gui/text/qfontengine.cpp

@@ -1344,13 +1344,13 @@ QByteArray QFontEngine::convertToPostscriptFontFamilyName(const QByteArray &fami
     return f;
 }
 
-/**
- * Some font engines like the windows font engine
- * can not reliable create outline paths
- */
+// Allow font engines (e.g. Windows) that can not reliably create
+// outline paths for distance-field rendering to switch the scene
+// graph over to native text rendering.
 bool QFontEngine::hasUnreliableGlyphOutline() const
 {
-    return false;
+    // Color glyphs (Emoji) are generally not suited for outlining
+    return glyphFormat == QFontEngine::Format_ARGB;
 }
 
 QFixed QFontEngine::lastRightBearing(const QGlyphLayout &glyphs, bool round)

src/plugins/platforms/windows/qwindowsfontengine.cpp

@@ -671,7 +671,7 @@ void QWindowsFontEngine::getGlyphBearings(glyph_t glyph, qreal *leftBearing, qre
 
 bool QWindowsFontEngine::hasUnreliableGlyphOutline() const
 {
-    return hasUnreliableOutline;
+    return hasUnreliableOutline || QFontEngine::hasUnreliableGlyphOutline();
 }
 
 qreal QWindowsFontEngine::minLeftBearing() const