Add input config for Google Stadia controller.

[dmccombs]

This adds Google Stadia controller support to InputAutoCfg. I'd like feedback though on how I've defined buttons.

Here's what a Stadia controller looks like. It works as a normal HID joystick when plugged in via a USB-C (to USB-A in my case) cable. I'm testing on Linux, I'd be curious if this works correctly on Windows or Mac depending how the controller identifies.

First, the straight forward ones:

• The Dpad is mapped to the Dpad
• The center N64 analog stick is mapped to the left analog stick on the Stadia
• The C Button Dpad is mapped to the right analog stick on the Stadia
• Start is mapped to the center Stadia button
• The L and R shoulder triggers are mapped to the Left/Right bumper buttons on the Stadia

The ones I'm unsure of:

• I've mapped the Z Trig to the Y button (the top button on the right side) on the Stadia for lack of a better idea
• I've mapped A and B to the buttons labeled as A and B on the Stadia, though that flips where they are compared to a real N64 controller. I thought about reversing them, but decided to map them to the buttons they're labeled as to avoid confusion.

I've left the X, Menu, Capture, Google Assistant, and Options buttons unused, along with the Left and Right analog triggers. Maybe one of these would make more sense for the Z Trig?

I also don't know if the rumble of the Stadia can be controlled in some way by Mupen64, if that's what Rumplepak switch is intended for.

Once I get some feedback on any changes to the mapping, I'll add text to the README describing this controller like the others.

[dmccombs]

@richard42 any thoughts on the button mapping, especially for Z Trig? Could I and should I map the analog L and/or R shoulder triggers to that? I'll open another PR with the README.md and possibly an update for that once I hear back.

[richard42]

It would be nice to map the screenshot function to the Capture button. There is currently no way to do that with the InputAutoCfg functionality, though you can achieve the same effect on a particular system by mapping the screenshot command to a particular button number on a particular joystick number.

I don't know if haptic feedback (rumble) is supported by your driver for the Stadia controller, but you can find out by looking through the console log after starting up Mupen64Plus with this controller plugged in. If so, it would be good to map the mempak/rumblepak switches.

The Z trigger should probably be mapped to the L2 left trigger.