Use Qt's own glyph cache with the freetype engine.

This fixes rendering in the OpenGL paint engine,
simplifies our internal code paths and also produces
nicer text for non-90 rotated glyphs

Change-Id: Ib206065cd486ac29067f9ebf4a0d74189a673f1e
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/gui/text/qfontengine_ft.cpp

@@ -631,8 +631,7 @@ QFontEngineFT::QFontEngineFT(const QFontDef &fd)
 #endif
     defaultFormat = Format_None;
     embeddedbitmap = false;
-    const QByteArray env = qgetenv("QT_NO_FT_CACHE");
-    cacheEnabled = env.isEmpty() || env.toInt() == 0;
+    cacheEnabled = qEnvironmentVariableIsSet("QT_USE_FT_CACHE");
     m_subPixelPositionCount = 4;
 }