Use truncate only for subpixel boundingBox x position

Use truncate only for subpixel boundingBox x position

Since Lion, Core Text starts to return fractional values for
x origin in the glyph bounding box. To get correct alignment
we need to make it integer, it seems that round will cut certain
pixels (x = 0.6 will be round to 1, then that glyph will be moved
too much to the left in image glyph cache). Reverting 4297b85a
appears to work fine on previous version of Mac OS X as well.

This change will not affect Windows (DirectWrite) and FreeType
font engines since they both return integer values for that.

Change-Id: I830f5b0bd27cad3a3a84b157da428134fb979703
Reviewed-by: Eskil
Reviewed-on: http://codereview.qt.nokia.com/1667
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>