Add a key to toggle map block bounds (#11172)

It's often useful to know where the map block boundaries are for doing server admin work and the like.
Adds three modes: single mapblock, range of 5, and disabled.

src/client/hud.h

@@ -59,6 +59,9 @@ public:
 			Inventory *inventory);
 	~Hud();
 
+	void toggleBlockBounds();
+	void drawBlockBounds();
+
 	void drawHotbar(u16 playeritem);
 	void resizeHotbar();
 	void drawCrosshair();
@@ -125,6 +128,14 @@ private:
 
 	scene::SMeshBuffer m_rotation_mesh_buffer;
 
+	enum BlockBoundsMode
+	{
+		BLOCK_BOUNDS_OFF,
+		BLOCK_BOUNDS_CURRENT,
+		BLOCK_BOUNDS_ALL,
+		BLOCK_BOUNDS_MAX
+	} m_block_bounds_mode = BLOCK_BOUNDS_OFF;
+
 	enum
 	{
 		HIGHLIGHT_BOX,