backends/libinput: Fix tablet cursor going out of bound
The cursor because of the specification of the libinput API (see https://wayland.freedesktop.org/libinput/doc/latest/tablet-support.html#out-of-bounds-motion-events) could be transported outside the bounderies, which cause problem with tiling. This merge request want to address this bug by using a std::clamp() the result.
Edited by Matteo Di Noia