gst-plugin: Try vp8 if plugin wasn't asked explicitly

It used to be the default behaviour but it was changed
with the following commit:

28616c7cSigned-off-by: Snir Sheriber <ssheribe@redhat.com>
Acked-by: Kevin Pouget <kpouget@redhat.com>

src/gst-plugin.cpp

@@ -524,6 +524,7 @@ SPICE_STREAMING_AGENT_PLUGIN(agent)
 {
     const std::string gst_prefix = "gst.";
     auto options = agent->Options();
+    bool registered = false;
 
     gst_init(nullptr, nullptr);
 
@@ -537,8 +538,15 @@ SPICE_STREAMING_AGENT_PLUGIN(agent)
 
             plugin->ParseOptions(agent->Options(), codec_name, value);
             agent->Register(plugin);
+            registered = true;
         }
     }
 
+    if (!registered) {
+        auto plugin = std::make_shared<GstreamerPlugin>();
+        plugin->ParseOptions(agent->Options(), "vp8", "auto");
+        agent->Register(plugin);
+    }
+
     return true;
 }