Mouse events position were unsigned (so negative values could not be
handled properly).
To avoid issues with negative values, mouse events outside the device
screen were ignored (commit a7fe9ad779).
But as a consequence, drag&drop were "broken" if the "drop" occurred
outside the device screen.
Instead, use signed 32-bits to store the position, and forward events
outside the device screen.
Fixes <https://github.com/Genymobile/scrcpy/issues/357>.
