Both the spline curves and (most of) the predefines curves are defined
as having start value 0.0 and end value 1.0. The spline and In/OutBack
functions would sometimes not produce that result precisely, so code
could not reliably depend on expressions like (easedValue < 1.0)
becoming false. Fix by explicitly handling endpoints.
Fixes: QTBUG-76781
Fixes: QTBUG-72630
Change-Id: I21be43af469a76c090154bffef8406a9baf2d0b1
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>