[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/tests/mouse_test.c

@@ -241,7 +241,12 @@ void app_mouseTrace(const char * file, unsigned int line,
   (void)format;
 }
 
-bool purespice_mouseMotion(int32_t x, int32_t y)
+bool lgInput_available(void)
+{
+  return g_params.useSpiceInput;
+}
+
+bool lgInput_mouseMotion(int32_t x, int32_t y)
 {
   push(EV_MOTION, x, y, false);
   return true;