api: get_streams (#886)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change
MSE method of streaming video needs the dimensions up front. This new
api delivers that. Also provides an easier way to determine which
screens are available and a way to discover port number.

## Testing
See pending changes in substream project

src/spice2x/api/stream_server.h

@@ -11,6 +11,9 @@
 
 namespace api {
 
+    // 0 while no stream server is listening, so the API can tell clients not to look for one
+    unsigned short stream_server_port();
+
     class StreamServer {
     public: