hidapi_steamdeck: severe input lag if vsync enabled

[jorio]

I maintain (Linux, OpenGL, non-Steam) games that run great on the Steam Deck with SDL 2.28.5.

Upon upgrading to SDL 2.30.4, I noticed severe input lag with the Steam Deck's built-in gamepad (joystick name "Steam Deck", GUID 0300f617de2800000512000000036800).

Each button press takes about half a second to register. This occurs when vsync is enabled in the game; input lag disappears if you turn vsync off.

The input lag also disappears if you get the game to use the second joystick ("Steam Virtual Gamepad", GUID 030079f6de280000ff11000001000000). In SDL 2.28.5, this is the only detected joystick.

The issue can be mitigated with SDL_SetHint(SDL_HINT_JOYSTICK_HIDAPI_STEAMDECK, "0"), which reverts to SDL 2.28.5's behavior.