MovementSpeedModifierComponent (#721)

* movement modifier system

* update default exposedata values

* changing base speed marks as dirty

* formatting

* movement speed modifier component

* Moves slowdown component to the prototype

AIController respects slowdown

* reset dirty flag

Content.Server/GameObjects/Components/Movement/MovementSpeedModifierComponent.cs

@@ -0,0 +1,68 @@
+
+using Robust.Shared.GameObjects;
+
+namespace Content.Server.GameObjects.Components.Movement
+{
+    [RegisterComponent]
+    public class MovementSpeedModifierComponent : Component
+    {
+        public override string Name => "MovementSpeedModifier";
+
+        private float _cachedWalkSpeedModifier = 1.0f;
+        public float WalkSpeedModifier
+        {
+            get
+            {
+                RecalculateMovementSpeedModifiers();
+                return _cachedWalkSpeedModifier;
+            }
+        }
+        private float _cachedSprintSpeedModifier;
+        public float SprintSpeedModifier
+        {
+            get
+            {
+                RecalculateMovementSpeedModifiers();
+                return _cachedSprintSpeedModifier;
+            }
+        }
+
+        /// <summary>
+        ///     set to warn us that a component's movespeed modifier has changed
+        /// </summary>
+        private bool _movespeedModifiersNeedRefresh = true;
+
+        public void RefreshMovementSpeedModifiers()
+        {
+            _movespeedModifiersNeedRefresh = true;
+        }
+
+        /// <summary>
+        ///     Recalculate movement speed with current modifiers, or return early if no change
+        /// </summary>
+        private void RecalculateMovementSpeedModifiers()
+        {
+            {
+                if (!_movespeedModifiersNeedRefresh)
+                    return;
+                var movespeedModifiers = Owner.GetAllComponents<IMoveSpeedModifier>();
+                float walkSpeedModifier = 1.0f;
+                float sprintSpeedModifier = 1.0f;
+                foreach (var component in movespeedModifiers)
+                {
+                    walkSpeedModifier *= component.WalkSpeedModifier;
+                    sprintSpeedModifier *= component.SprintSpeedModifier;
+                }
+                _cachedWalkSpeedModifier = walkSpeedModifier;
+                _cachedSprintSpeedModifier = sprintSpeedModifier;
+            }
+            _movespeedModifiersNeedRefresh = false;
+        }
+    }
+
+    interface IMoveSpeedModifier
+    {
+        float WalkSpeedModifier { get; }
+        float SprintSpeedModifier { get; }
+    }
+}