A scroll event might be produced when a mouse button is pressed (for
example when scrolling while selecting a text). For consistency, pass
the actual buttons state (instead of 0).
In practice, it seems that this use case does not work properly with
Android event injection, but it will work with HID mouse.
This aims to make the key/mouse processors independent of the "screen",
by processing scrcpy-specific input events instead of SDL events.
In particular, these scrcpy events are not impacted by any UI window
scaling or rotation (contrary to SDL events).
