Change antialiasing method for QML2.

Change antialiasing method for QML2.

Since multisampling can require a lot of memory, and might not
be supported on some hardware, turn off multisampling and
implement antialiasing in the vertex shader instead. The
alternative method of antialiasing is implemented for Rectangle,
Image, BorderImage and AnimatedImage, and must be explicitly
enabled by setting the new antialiasing property.

Task-number: QTBUG-26268
Change-Id: I39a93d978658a494bf51e9f0fd02d8414eb8be12
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>