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