[gamepad] Allow standard-mapped devices to have extra buttons or axes & allow multiple mappings per device

Hi all,

I recently was asked to review the Gamepad API draft specification.  My
background is games though I've done some scientific computing with
alternate input devices too.

http://chadaustin.me/2014/10/the-gamepad-api/

I'd like to suggest two simple changes to the Gamepad API.  In tandem,
these APIs would allow applications to have automatic compatibility with a
broader range of devices.

The changes are:

1. Allow standard-mapped devices to have extra buttons or axes
2. Add support for multiple standard mappings per device.

Together, these changes would allow a simple application that only needs a
d-pad and a few buttons to work automatically with a Wiimote, XInput
device, and a racing wheel (many racing wheels have d-pads on them).

Even without the proposal #2, proposal #1 would allow optional access to
extra capabilities on a device without having to sacrifice the standard
mapping.

This feels noncontroversial to me because mappings only assign meaning to
otherwise neutral indices in the buttons[] and axes[] arrays.  For a given
device, there can be many such meanings.

-- 
Chad Austin
http://chadaustin.me