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>.
The video screen size on the client may differ from the real device
screen size (e.g. the video stream may be scaled down). As a
consequence, mouse events must be scaled to match the real device
coordinates.
For this purpose, make the client send the video screen size along with
the absolute pointer location, and the server scale the location to
match the real device size before injecting mouse events.
