[Feature Request] SDL3: left/right half of touchpad as separate buttons

The touchpad on DualShock 4 / DualSense has left and right regions as separate buttons in other applications, but SDL2 and 3 on RA currently sees one button for the whole pad.
SDL3 reports 2 fingers and their position, so we can use that to determine which half of the surface is being pressed, and use those as bindable buttons to mimic behavior seen elsewhere.
I'm only interested in 2 buttons, but we could also use a 4-button grid or more, probably a harder implementation.
The existing SDL_GAMEPAD_BUTTON_TOUCHPAD can't coexist, this can be disabled with the same toggle that enables the feature.
The tricky part is adding new buttons on top of the SDL_GAMEPAD_BUTTON_COUNT, if that increases then everyone's mappings will change.
I think this API is supported in SDL2 as well as SDL3, maybe other drivers (mfi?) Do we want to support this more generically?

I have a proof of concept working in SDL3, but I'd like to see a discussion about this before proceeding.

Start by reviewing RetroArch's SDL2/SDL3 gamepad input handling and the existing SDL_GAMEPAD_BUTTON_TOUCHPAD mapping. Check the SDL3 two-finger position data and the SDL_GAMEPAD_BUTTON_COUNT compatibility concern, then determine how the toggle should switch between one touchpad button and two bindable regions. Done means the behavior and cross-version scope are resolved without changing existing mappings unexpectedly.