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.

VisualC/SDL/SDL.vcxproj

@@ -280,6 +280,7 @@
     <ClInclude Include="..\..\include\SDL3\SDL_opengles2_gl2ext.h" />
     <ClInclude Include="..\..\include\SDL3\SDL_opengles2_gl2platform.h" />
     <ClInclude Include="..\..\include\SDL3\SDL_opengles2_khrplatform.h" />
+    <ClInclude Include="..\..\include\SDL3\SDL_pen.h" />
     <ClInclude Include="..\..\include\SDL3\SDL_pixels.h" />
     <ClInclude Include="..\..\include\SDL3\SDL_platform.h" />
     <ClInclude Include="..\..\include\SDL3\SDL_platform_defines.h" />
@@ -507,6 +508,7 @@
     <ClCompile Include="..\..\src\events\SDL_events.c" />
     <ClCompile Include="..\..\src\events\SDL_keyboard.c" />
     <ClCompile Include="..\..\src\events\SDL_mouse.c" />
+    <ClCompile Include="..\..\src\events\SDL_pen.c" />
     <ClCompile Include="..\..\src\events\SDL_quit.c" />
     <ClCompile Include="..\..\src\events\SDL_touch.c" />
     <ClCompile Include="..\..\src\events\SDL_windowevents.c" />

VisualC/SDL/SDL.vcxproj.filters

@@ -294,6 +294,9 @@
     <ClInclude Include="..\..\include\SDL3\SDL_opengles2_khrplatform.h">
       <Filter>API Headers</Filter>
     </ClInclude>
+    <ClInclude Include="..\..\include\SDL3\SDL_pen.h">
+      <Filter>API Headers</Filter>
+    </ClInclude>
     <ClInclude Include="..\..\include\SDL3\SDL_pixels.h">
       <Filter>API Headers</Filter>
     </ClInclude>
@@ -921,6 +924,9 @@
     <ClCompile Include="..\..\src\events\SDL_mouse.c">
       <Filter>events</Filter>
     </ClCompile>
+    <ClCompile Include="..\..\src\events\SDL_pen.c">
+      <Filter>events</Filter>
+    </ClCompile>
     <ClCompile Include="..\..\src\events\SDL_quit.c">
       <Filter>events</Filter>
     </ClCompile>