swipeTracker: Immediately flip delta in RTL locales

swipeTracker: Immediately flip delta in RTL locales

The delta is currently flipped in a shared code path, after appending
the unmodified value to the event history. As a result, the calculated
final velocity for touchpad gestures is wrong in RTL locales.

Fix this by modifying the delta immediately when handling the events,
so that all other code operates on the correct values.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8840
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3967>
(cherry picked from commit a02835aacadbc14dc4350add3bae99c70e38f1b0)