The decision to not send motion events when no click is pressed is
specific to Android mouse injection. Other mouse processors (e.g. for
HID mouse) will need to receive all events.
The default mouse injection works in absolute mode: it forwards clicks
at a specific position on screen.
To support HID mouse, add a flag to indicate that the mouse processor
works in relative mode: it forwards mouse motion vectors, without any
absolute reference to the screen.
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.
Pass scrcpy input events instead of SDL input events to mouse
processors.
These events represent exactly what mouse processors need, abstracted
from any visual orientation and scaling applied on the SDL window.
This makes the mouse processors independent of the "screen" instance,
and the implementation source code independent of the SDL API.
