Add a renderer selection option for macOS

There are numerous conflicting reports about which Mac renderer works best (even on the same systems!). I give up, just let the users figure it out.

app/streaming/video/decoder.h

@@ -36,6 +36,7 @@ typedef struct _VIDEO_STATS {
 typedef struct _DECODER_PARAMETERS {
     SDL_Window* window;
     StreamingPreferences::VideoDecoderSelection vds;
+    StreamingPreferences::RendererSelection renderer;
 
     int videoFormat;
     int width;