SDL Adds Default Steam Controller Mapping For Linux

It seems like it would have been done long ago, but upstream SDL2 now finally has out-of-the-box Steam Controller mappings for Linux and iOS.

While Steam Controller mappings could be made individually via configuring the controller with Steam Big Picture Mode or from the Linux desktop with the controllermaputility, the upstream SDL code now has default entries for the Steam Controller for both Apple and Linux for benefiting games making use of SDL2's game controller support.

The mappings landed with this commit by Valve's Sam Lantinga.