[client] input: add display server behaviour tests

Extract input policy from the Wayland and X11 protocol handlers.

Exercise both implementations against literal Wayland event traces.

client/displayservers/input.h

@@ -0,0 +1,44 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2026 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#ifndef _H_LG_DS_INPUT_
+#define _H_LG_DS_INPUT_
+
+#include <stdbool.h>
+
+typedef struct LG_DSInputSink
+{
+  void (*position)(void * opaque, double x, double y);
+  void (*relative)(void * opaque, double x, double y,
+      double rawX, double rawY);
+  void (*button)(void * opaque, unsigned int button, bool pressed);
+  void (*wheel)(void * opaque, double motion);
+  void (*key)(void * opaque, int scancode, bool pressed);
+  void (*text)(void * opaque, const char * text);
+  void (*modifiers)(void * opaque, bool ctrl, bool shift, bool alt,
+      bool super);
+  void (*leds)(void * opaque, bool numLock, bool capsLock,
+      bool scrollLock);
+  void (*enter)(void * opaque, bool entered);
+  void (*focus)(void * opaque, bool focused);
+}
+LG_DSInputSink;
+
+#endif