[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/include/interface/transport.h

@@ -27,6 +27,7 @@
 
 #include "common/framebuffer.h"
 #include "common/types.h"
+#include "interface/input.h"
 
 #define LG_TRANSPORT_MAX_DAMAGE_RECTS LG_MAX_FRAME_DAMAGE_RECTS
 
@@ -244,6 +245,10 @@ typedef struct LG_TransportOps
   void (*disconnect)(LG_Transport * transport);
   bool (*sessionValid)(LG_Transport * transport);
   bool (*supportsDMA)(LG_Transport * transport);
+  /* Queried after connect. The returned operations and opaque value remain
+   * valid until disconnect; NULL indicates that this session has no input. */
+  const LG_InputOps *(*getInputOps)(LG_Transport * transport,
+      void ** opaque);
   bool (*attachRenderer)(LG_Transport * transport,
       const LG_RendererInterop * interop);
   void (*detachRenderer)(LG_Transport * transport);