Add option for disabling all input (view-only mode) in streaming session

* By default this option is set to `false` and can be enabled using the
  new `-viewonly`/`viewonly` command-line/config options.
* When enabled, none of the input modules will be initialised, making
  the streaming session view-only, i.e. no input is sent to host.

To facilitate implementation, SDL initialisation with `sdl_init()` was moved
a bit earlier in the code to decouple from input initialisation.

docs/README.pod

@@ -149,6 +149,10 @@ Try streaming if GFE version or options are unsupported
 
 Send quit app request to remote after quitting session
 
+=item B<-viewonly>
+
+Disable all input processing (view-only mode)
+
 =item B<-verbose>
 
 Enable verbose output