Add option to use native rasterizer for SceneGraph text

For old-style (desktop components) apps using QML 2 on regular density
displays, distance field text will look out of place. We introduce
an option to use the native rasterizer instead if you would rather
have native look and feel than scalable text items.

Change-Id: Idb38e3c89f2deab9ae1963357c6c5fb235ddeab8
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>