[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/transports/LGMP/lgmp.c

@@ -1327,6 +1327,13 @@ static LG_TransportStatus lgmp_controlStatus(LG_Transport * this,
     LG_TRANSPORT_OK : LG_TRANSPORT_UNAVAILABLE;
 }
 
+static const LG_InputOps * lgmp_getInputOps(LG_Transport * this,
+    void ** opaque)
+{
+  *opaque = NULL;
+  return NULL;
+}
+
 const LG_TransportOps LGT_LGMP =
 {
   .name           = "lgmp",
@@ -1337,6 +1344,7 @@ const LG_TransportOps LGT_LGMP =
   .disconnect     = lgmp_disconnect,
   .sessionValid   = lgmp_sessionValid,
   .supportsDMA    = lgmp_supportsDMA,
+  .getInputOps    = lgmp_getInputOps,
   .attachRenderer = lgmp_attachRenderer,
   .detachRenderer = lgmp_detachRenderer,
   .nextFrame      = lgmp_nextFrame,