slider: Don't preserve paths after filling

slider: Don't preserve paths after filling

Prior to 6fd0aac86 the paths were preserved when filling to allow
stroking the path afterwards. The stroke() then would have cleared the
path. Then when the stroking was dropped in that commit, a second fill()
call, this time without preserving the path was added, but missed from
the end arc of the bar level. This then would leak the path to the
slider handle.

Fix this by always using fill() instead of fillPreserve().

Fixes: 6fd0aac86 ("slider: Remove border drawing code add hover style")
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3817>
(cherry picked from commit 95b407b57f6334cc1895d838c0b5fb7c42ff9c16)