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_format.cpp

@@ -67,4 +67,18 @@ namespace api {
 
         return nullptr;
     }
+
+    std::vector<std::pair<std::string, std::string>> stream_formats() {
+        std::vector<std::pair<std::string, std::string>> formats;
+
+#ifdef SPICE_JPEG
+        formats.emplace_back("mjpeg", "/stream.mjpg");
+#endif
+
+#ifdef SPICE_H264
+        formats.emplace_back("h264", "/stream.h264");
+#endif
+
+        return formats;
+    }
 }