shared_ptr -> std::shared_ptr

This is one of the first commits in a plan to remove all `using namespace std;` lines in the entire codebase as it is considered anti-pattern today.

Minecraft.World/DownfallMixerLayer.h

@@ -5,10 +5,10 @@
 class DownfallMixerLayer : public Layer
 {
 private:
-	shared_ptr<Layer> downfall;
+	std::shared_ptr<Layer> downfall;
 	int layer;
 
 public:
-	DownfallMixerLayer(shared_ptr<Layer> downfall, shared_ptr<Layer> parent, int layer);
+	DownfallMixerLayer(std::shared_ptr<Layer> downfall, std::shared_ptr<Layer> parent, int layer);
 	intArray getArea(int xo, int yo, int w, int h);
 };