Added a faster subpixel antialiasing material for distance field text.

Based on two texture samples instead of five.
Can be enabled with qmlscene --text-subpixel-antialiasing-lowq

Change-Id: I726f73d812b93aa9ca38ce142d1e97b9a40d200a
Reviewed-on: http://codereview.qt.nokia.com/2861


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Yoann Lopes <yoann.lopes@nokia.com>