Export QOpenGLTextureCache

because it can be useful outside QtGui. The function
QOpenGLTextureCache::bindTexture gives a very convenient way to get a
texture from an image.

Change-Id: I2e22c0a3a8f1f307d0b558280043f726e3d8093a
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/gui/opengl/qopengltexturecache_p.h

@@ -73,7 +73,7 @@ private:
     QOpenGLSharedResourceGuard *m_resource;
 };
 
-class QOpenGLTextureCache : public QOpenGLSharedResource
+class Q_GUI_EXPORT QOpenGLTextureCache : public QOpenGLSharedResource
 {
 public:
     static QOpenGLTextureCache *cacheForContext(QOpenGLContext *context);