Dynamic controller assignment

[imsys]

This was mainly coded by Gemini, I mostly reviewed and tested it.

This fixes #2955. I tested with cart 893, note that this test cart is positioned as:

P1	P3
P2	P4

Before this patch, when I try to play on my PC with a Xbox controller, it is assigned to P3. If I plug another controller, it gets assigned to P4.

With this patch I also ested that:
The first player to that moves or press any button is the 1st player. If I turn it off and on again, it also keeps being the 1st player.

This will probably have a merge conflict with #2952

Following is the comments by Gemini:

Key Changes:

Increased Controller Support: Expanded the internal controller ports from 4 to 16. This ensures that even if you have multiple "ghost" devices or adapters at low SDL indices, your active controller will still be detected and opened.

Assignment by First Use: Introduced a mapping system that tracks the registration order of controllers. A controller is only assigned to a player slot (1-4) once it shows some activity (button press or stick movement).

Active Removal Handling: Corrected the hotplug logic to properly identify which controller is being disconnected using its unique SDL Instance ID, ensuring assigned slots are freed if a controller is unplugged.

Robust Mapping: Initialized all player slots as unassigned at startup. The first controller to send input becomes Player 1, the second becomes Player 2, and so on.

Details:

• Added SDL_JoystickID mapping[TIC_GAMEPADS] to the platform.gamepad structure.
• Implemented isControllerActive() to detect button presses or axis movements exceeding a deadzone threshold.
• Updated processGamepad() to perform a two-phase check: first, mapping unassigned active controllers to available slots; then, reading input from the mapped controllers.
• Fixed SDL_CONTROLLERDEVICEREMOVED to use Instance IDs instead of indices, which is the correct behavior for this SDL event.