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/DownfallLayer.h

@@ -5,6 +5,6 @@
 class DownfallLayer : public Layer
 {
 public:
-	DownfallLayer(shared_ptr<Layer>parent);
+	DownfallLayer(std::shared_ptr<Layer>parent);
 	intArray getArea(int xo, int yo, int w, int h);
 };