This changes behaviour, however I think it's a net positive:
* Most games using Steam Input or SDL2 to read game controllers have
this behaviour, so users have come to expect it.
* This module is better at giving meaningful names to buttons on
common controller, and assigning axes consistently.
* Button/axis mapping files using a widely-used format are supported.
* The old behaviour is still available with `-joystickprovider sdljoy`
if anyone wants it.
The new option for controller mapping files is in the general OSD
options rather than SDL options as it can be extended to DirectInput in
the future.
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