API for pressure-sensitive pens + XInput2/Wayland

This patch adds an API for querying pressure-
sensitive pens, cf. SDL_pen.h:
- Enumerate all pens
- Get pen capabilities, names, GUIDs
- Distinguishes pens and erasers
- Distinguish attached and detached pens
- Pressure and tilt support
- Rotation, distance, throttle wheel support
  (throttle wheel untested)
- Pen type and meta-information reporting
  (partially tested)

Pen event reporting:
- Three new event structures: PenTip, PenMotion, and
  PenButton
- Report location with sub-pixel precision
- Include axis and button status, is-eraser flag

Internal pen tracker, intended to be independent
of platform APIs, cf. SDL_pen_c.h:
- Track known pens
- Handle pen hotplugging

Automatic test:
- testautomation_pen.c

Other features:
- XInput2 implementation, incl. hotplugging
- Wayland implementation, incl. hotplugging
- Backward compatibility: pen events default to
  emulating pens with mouse ID SDL_PEN_MOUSEID
- Can be toggled via SDL_HINT_PEN_NOT_MOUSE
- Test/demo program (testpen)
- Wacom pen feature identification by pen ID

Acknowledgements:
- Ping Cheng (Wacom) provided extensive feedback
  on Wacom pen features and detection so that
  hopefully untested Wacom devices have a
  realistic chance of working out of the box.

include/SDL3/SDL_hints.h

@@ -1220,6 +1220,40 @@ extern "C" {
  */
 #define SDL_HINT_MOUSE_AUTO_CAPTURE    "SDL_MOUSE_AUTO_CAPTURE"
 
+/**
+ *  Treat pen movement as separate from mouse movement
+ *
+ *  By default, pens report both ::SDL_MouseMotionEvent and ::SDL_PenMotionEvent updates
+ *  (analogously for button presses).  This hint allows decoupling mouse and pen updates.
+ *
+ *  This variable toggles between the following behaviour:
+ *    "0"       - (Default) Pen acts as a mouse with mouse ID ::SDL_PEN_MOUSEID.
+ *                Use case: client application is not pen aware, user wants to
+ *                use pen instead of mouse to interact.
+ *    "1"       - Pen reports mouse clicks and movement events but does not update
+ *                SDL-internal mouse state (buttons pressed, current mouse location).
+ *                Use case: client application is not pen aware, user frequently
+ *                alternates between pen and "real" mouse.
+ *    "2"       - Pen reports no mouse events.
+ *                Use case: pen-aware client application uses this hint to allow user to
+ *                toggle between pen+mouse mode ("2") and pen-only mode ("1" or "0").
+ */
+#define SDL_HINT_PEN_NOT_MOUSE    "SDL_HINT_PEN_NOT_MOUSE"
+
+/**
+ *  Pen mouse button emulation triggers only when the pen touches the tablet surface
+ *
+ *    "0"       - The pen reports mouse button press/release immediately when the pen
+ *                button is pressed/released, and the pen tip touching the surface counts
+ *                as left mouse button press.
+ *    "1"       - (Default) Mouse button presses are sent when the pen first touches
+ *                the tablet (analogously for releases).  Not pressing a pen button
+ *                simulates mouse button 1, pressing the first pen button simulates
+ *                mouse button 2 etc.;  it is not possible to report multiple buttons
+ *                as pressed at the same time.
+ */
+#define SDL_HINT_PEN_DELAY_MOUSE_BUTTON    "SDL_HINT_PEN_DELAY_MOUSE_BUTTON"
+
 /**
  *  Tell SDL not to catch the SIGINT or SIGTERM signals.
  *