Add sprite fades for occluding sprites (#13267)

Content.Client/Sprite/FadingSpriteComponent.cs

@@ -0,0 +1,13 @@
+using Content.Shared.Sprite;
+
+namespace Content.Client.Sprite;
+
+/// <summary>
+/// The non-networked client-only component to track active <see cref="SpriteFadeComponent"/>
+/// </summary>
+[RegisterComponent, Access(typeof(SpriteFadeSystem))]
+public sealed class FadingSpriteComponent : Component
+{
+    [ViewVariables]
+    public float OriginalAlpha;
+}