Add GPU stat for number of depth copies per frame

Will be useful for evaluating #15700

GPU/Common/FramebufferManagerCommon.cpp

@@ -494,6 +494,8 @@ void FramebufferManagerCommon::BlitFramebufferDepth(VirtualFramebuffer *src, Vir
 		return;
 	}
 
+	gpuStats.numDepthCopies++;
+
 	int w = std::min(src->renderWidth, dst->renderWidth);
 	int h = std::min(src->renderHeight, dst->renderHeight);