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.

test/CMakeLists.txt

@@ -328,6 +328,7 @@ add_sdl_test_executable(testgles2 SOURCES testgles2.c)
 add_sdl_test_executable(testgles2_sdf NEEDS_RESOURCES TESTUTILS SOURCES testgles2_sdf.c)
 add_sdl_test_executable(testhaptic SOURCES testhaptic.c)
 add_sdl_test_executable(testhotplug SOURCES testhotplug.c)
+add_sdl_test_executable(testpen SOURCES testpen.c)
 add_sdl_test_executable(testrumble SOURCES testrumble.c)
 add_sdl_test_executable(testthread NONINTERACTIVE NONINTERACTIVE_TIMEOUT 40 SOURCES testthread.c)
 add_sdl_test_executable(testiconv NEEDS_RESOURCES TESTUTILS SOURCES testiconv.c)