bestsource: Mark video provider as slow

Looks like this wasn't clear enough, so let's make it clear to users
that BestVideoSource should only be used if absolutely necessary.

src/video_provider_manager.cpp

@@ -50,7 +50,7 @@ namespace {
 		{"Avisynth", CreateAvisynthVideoProvider, false},
 #endif
 #ifdef WITH_BESTSOURCE
-		{"BestSource", CreateBSVideoProvider, false},
+		{"BestSource (SLOW)", CreateBSVideoProvider, false},
 #endif
 	};
 }