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.

src/config.h

@@ -41,6 +41,7 @@ typedef struct _CONFIGURATION {
   bool fullscreen;
   bool unsupported;
   bool quitappafter;
+  bool viewonly;
   char* inputs[MAX_INPUTS];
   int inputsCount;
   enum codecs codec;