Allow user to override artwork

[icculus]

Right now they might prefer a different controller, or more likely fallback to xbox360 for something unrecognized, so it might be nice for them to be able to:

a) Load an overlay database via environment variable b) and/or be able to force a specific guid to use a specific art set (CONTROLLERIMAGE_DEVMAP=ab935235bed0245134:stadia or something).

[Raptor85]

aside from being able to do it from an environment variable it would be nice to be able to override it in the code as well to allow users to change glyphs from in-game, I guess you could create a "dummy" device with the load function that takes a device type string parameter for some common options but that feels like somewhat of a messy way to do it.

[icculus]

In code you can already do this. Instead of creating a ControllerImage_Device * by handing it an SDL_Gamepad or SDL_JoystickID, you can just request a specific artset:

ControllerImage_Device *imgdev = ControllerImage_CreateGamepadDeviceByIdString("xbox360");

(and of course, you can just load an overlay database by calling ControllerImage_AddData* a second time.)