Fix misaligned selection region with text when centered

If the text is centered, the x/y position in the selection QRectF may
be a multiple of 0.5 which is rounded up. This rounding causes
misalignment of the selection region with the text.

The alignment is fixed by using qFloor on the x and y components.

Task-number: QTBUG-34218
Task-number: QTBUG-34234
Change-Id: I4f2fadeb38602f62a93773c6e5faecf03b28069f
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
(cherry picked from qtbase/5d8a882c11201a29475c5ea71cfb76c9de6573f5)