add GameControllerDB mappings before initializing the SDL gamecontroller subsystem

[briaguya-ai]

controllers that don't have built in mappings and rely on gamecontrollerdb.txt mappings were firing SDL_JOYDEVICEADDED events at boot but not SDL_CONTROLLERDEVICEADDED events

The documentation for SDL_ControllerDeviceEvent states:

Joysticks that are supported game controllers receive both an SDL_JoyDeviceEvent and an SDL_ControllerDeviceEvent.

SDL will send CONTROLLERDEVICEADDED events for joysticks that are already plugged in during SDL_Init() and are recognized as game controllers.

i looked a bit and found we weren't loading gamecontrollerdb.txt until after initializing the game controller subsystem, so the "already plugged in" events were firing when the devices weren't recognized as game controllers yet.

this PR ensures SDL_GameControllerAddMappingsFromFile happens before the subsystem is initialized. i have verified this makes it so we get SDL_CONTROLLERDEVICEADDED events on boot.

relevant discourse post from someone with a similar issue: https://discourse.libsdl.org/t/deviced-added-events-for-devices-plugged-in-at-game-start/55039

[briaguya-ai]

confirmed working by cobeck on discord