QQuickTextInput: keep floating point precition when calculating offset

Without this patch, positionToRectangle will return a slightly
different rectangle than what ends up being drawn.

Change-Id: Ib1a3936f0fab393d6016d85d63547ec7f3036b7a
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>