[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/main.h

@@ -99,8 +99,6 @@ struct AppState
   int                  escapeAction;
   bool                 escapeHelp;
   struct ll          * bindings;
-  atomic_bool          keyDown[KEY_MAX];
-
   bool                 haveSrcSize;
   struct Point         windowPos;
   int                  windowW, windowH;