slider: Use correct handle size when calculating rightmost position

slider: Use correct handle size when calculating rightmost position

Using the ceiled radius for calculating the rightmost position the
handle can take on the slider was resulting in a small gap when the size
was not an integer value since the radius used when drawing the handle
is not ceiled.

This effectively reverts 38da479e which seems to have been the wrong fix
for the problem it was trying to solve. The problem presumably had been
caused by not considering the border that the handle still had at that
time.

The border issue was then later fixed by 3ddae9d8, without reverting
38da479e. Then in 6fd0aac8 support for drawing borders on the slider got
dropped, including the changes from 3ddae9d8, leaving us with just the
changes from 38da479e but now without borders on the handle.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8187
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3648>