Add a new stat, so we can see per game if the optimization has an effect

GPU/Vulkan/DrawEngineVulkan.cpp

@@ -1016,6 +1016,7 @@ void DrawEngineVulkan::DoFlush() {
 
 	gpuStats.numFlushes++;
 	gpuStats.numDrawCalls += numDrawInds_;
+	gpuStats.numVertexDecodes += numDrawVerts_;
 	gpuStats.numVertsSubmitted += vertexCountInDrawCalls_;
 
 	indexGen.Reset();