Fix QGLWidget::renderPixmap for raster engine on Mac

Fix QGLWidget::renderPixmap for raster engine on Mac

The QPixmap buffer is backed by QRasterPixmapData instead of
QMacPixmapData for the raster engine, thus we have to update
qt_mac_pixmap_get_base() and qt_mac_pixmap_get_bytes_per_line(),
so that QGLWidget can locate the offscreen buffer from a QPixmap.

Reviewed-by: Fabien Freling
(cherry picked from commit c5846314dfd80e7f7f32ba737f1884dcccbbd80d)

Change-Id: I2414222f8a59e02c778177d52ad9a6e0ff68668d
Reviewed-on: http://codereview.qt.nokia.com/123
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>