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>

app/src/screen.c

@@ -477,6 +477,7 @@ sc_screen_init(struct sc_screen *screen,
         .screen = screen,
         .kp = params->kp,
         .mp = params->mp,
+        .gp = params->gp,
         .mouse_bindings = params->mouse_bindings,
         .legacy_paste = params->legacy_paste,
         .clipboard_autosync = params->clipboard_autosync,