QFontEngine: Cache whether or not a font can be smoothly scaled

This will be used by QtQuick to correct a performance regression introduced by
592614ea3e -- QFontDatabase::isSmoothlyScalable is
quite computationally expensive; and now it is unconditionally expensive
regardless of the platform.

Change-Id: I82bfa65a963c6c3c276d574f2b379da4a9ba5b69
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/gui/text/qfontdatabase.cpp

@@ -948,8 +948,8 @@ QFontEngine *loadSingleEngine(int script,
                     return 0;
                 }
 
+                engine->isSmoothlyScalable = style->smoothScalable;
                 fontCache->insertEngine(key, engine);
-
                 return engine;
             }
         }
@@ -972,6 +972,7 @@ QFontEngine *loadSingleEngine(int script,
                 return 0;
             }
 
+            engine->isSmoothlyScalable = style->smoothScalable;
             fontCache->insertEngine(key, engine);
 
             if (Q_LIKELY(cacheForCommonScript && !engine->symbol)) {

src/gui/text/qfontengine.cpp

@@ -251,6 +251,7 @@ QFontEngine::QFontEngine(Type type)
     cache_cost = 0;
     fsType = 0;
     symbol = false;
+    isSmoothlyScalable = false;
 
     glyphFormat = Format_None;
     m_subPixelPositionCount = 0;

src/gui/text/qfontengine_p.h

@@ -283,6 +283,7 @@ public:
     uint cache_cost; // amount of mem used in bytes by the font
     uint fsType : 16;
     bool symbol;
+    bool isSmoothlyScalable;
     struct KernPair {
         uint left_right;
         QFixed adjust;