Add support for SDL3 controllers including Steam Controller 2

Expected behavior of the wanted feature

Steam Controller, which provides output via the SDL3 API (and does not output Xinput or SDL 2) , should be able to be used to provide input to mpv

Alternative behavior of the wanted feature

Steam Controller cannot be used to provide input when in gamepad mode unless mpv is added as a non-steam game (if this matters to you, the reason i find this a problem is that i dont want my steam status to be showing as in-game in mpv, because non-steam games cannot be marked private)

Log File

i am using a fork called mpv.net and tbh i couldnt figure out how to get the log file, but this is a feature request, not a bug report. and i checked to make sure this has not been implemented in mpv already so that shouldnt be an issue
if there is something from the log file that would still be useful, please help me figure out where i am supposed to find it after running mpvnet --log-file=output.txt
i apologize for being a low-tech chud, would contribute this myself if i knew how

Sample Files

No response

The issue does not name files or tests; start by locating mpv's existing controller input handling and any SDL2 or XInput support. Add SDL3 support for Steam Controller input, including Steam Controller 2, and verify that gamepad input works without adding mpv as a non-Steam game.