pen: fix buttons incorrectly mapped on linux backends

[LordCarve]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

On linux backends (both X11 and Wayland) the pen-to-mouse remapping applied in SDL_pen.c confused RMB with MMB by hardcoding RMB to button 2 and MMB to button 3 (windows-canon).
Erroneous flows:

• Button 2 (unix MMB) passed as pen button 1 mapped to SDL_BUTTON_RIGHT = 3 (unix RMB).
• Button 3 (unix RMB) passed pen button 2 mapped to SDL_BUTTON_MIDDLE = 2 (unix MMB).

Since which pen button corresponds to which mouse button is backend-dependent, this information should be passed from backend into SDL_SendPenButton() and so its signature has been extended with Uint8 mouse_button (with button renamed to pen_button for clarity).

Now in pen-as-mouse scenarios backend has full control over which mouse button is used in mouse events, and it leverages that to pass the correct per-backend mouse buttons.

As a result, the bug of RMB<->MMB being mixed-up on linux backends is resolved.

For other backends I maintained the exact pen-to-mouse button mappings as they were before (including the edge-case of android's pen button 5 sending a pen event but explicitly not sending a mouse event). This is to avoid regression because I can't test any other backends.

I tested X11 and Wayland, and this addresses the underlying issue.

Existing Issue(s)

Fixes #14788