Handle SDL gamepad events

Introduce a gamepad processor trait, similar to the keyboard processor
and mouse processor traits.

Handle gamepad events received from SDL, convert them to scrcpy-specific
gamepad events, and forward them to the gamepad processor.

Further commits will provide AOA and UHID implementations of the gamepad
processor trait.

PR #5270 <https://github.com/Genymobile/scrcpy/pull/5270>

Co-authored-by: Luiz Henrique Laurini <luizhenriquelaurini@gmail.com>