[client] input: abstract guest input backends

Introduce LG_InputOps and route keyboard and mouse input through the
active video transport when it provides an input backend.

Keep SPICE as the fallback and force it while the SPICE display is
active. Add atomic shared/exclusive locking for safe backend changes.

client/src/input_spice.c

@@ -0,0 +1,82 @@
+/**
+ * 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
+ */
+
+#include "input.h"
+#include "kb.h"
+
+#include <purespice.h>
+#include <stddef.h>
+
+static bool spiceSupports(void * opaque, LG_InputSupport support)
+{
+  return false;
+}
+
+static bool spiceKeyDown(void * opaque, int key)
+{
+  const uint32_t ps2 = linux_to_ps2[key];
+  return !ps2 || purespice_keyDown(ps2);
+}
+
+static bool spiceKeyUp(void * opaque, int key)
+{
+  const uint32_t ps2 = linux_to_ps2[key];
+  return !ps2 || purespice_keyUp(ps2);
+}
+
+static bool spiceKeyboardLEDs(void * opaque, bool numLock, bool capsLock,
+    bool scrollLock)
+{
+  const uint32_t modifiers =
+    (scrollLock ? 1 /* SPICE_SCROLL_LOCK_MODIFIER */ : 0) |
+    (numLock    ? 2 /* SPICE_NUM_LOCK_MODIFIER    */ : 0) |
+    (capsLock   ? 4 /* SPICE_CAPS_LOCK_MODIFIER   */ : 0);
+
+  return purespice_keyModifiers(modifiers);
+}
+
+static bool spiceMouseMotion(void * opaque, int32_t x, int32_t y)
+{
+  return purespice_mouseMotion(x, y);
+}
+
+static bool spiceMousePress(void * opaque, unsigned int button)
+{
+  return purespice_mousePress(button);
+}
+
+static bool spiceMouseRelease(void * opaque, unsigned int button)
+{
+  return purespice_mouseRelease(button);
+}
+
+const LG_InputOps LGI_Spice =
+{
+  .name          = "SPICE",
+  .supports      = spiceSupports,
+  .keyDown       = spiceKeyDown,
+  .keyUp         = spiceKeyUp,
+  .keyboardLEDs  = spiceKeyboardLEDs,
+  .mouseMotion   = spiceMouseMotion,
+  .mousePosition = NULL,
+  .mousePress    = spiceMousePress,
+  .mouseRelease  = spiceMouseRelease,
+  .reset         = NULL,
+};