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.
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.
