Add basic support for drawing circles.

Conflicts:
	osu.Game.Modes.Taiko/Objects/Drawable/DrawableTaikoHitObject.cs

osu.Game.Modes.Taiko/Objects/Drawable/DrawableTaikoHitObject.cs

@@ -0,0 +1,62 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using OpenTK.Graphics;
+using osu.Framework.Graphics;
+using osu.Game.Modes.Objects.Drawables;
+using osu.Game.Modes.Taiko.Judgements;
+using osu.Game.Modes.Taiko.Objects.Drawable.Pieces;
+
+namespace osu.Game.Modes.Taiko.Objects.Drawable
+{
+    public abstract class DrawableTaikoHitObject : DrawableHitObject<TaikoHitObject, TaikoJudgementInfo>
+    {
+        /// <summary>
+        /// The colour used for various elements of this DrawableHitObject.
+        /// </summary>
+        public virtual Color4 AccentColour { get; }
+
+        protected DrawableTaikoHitObject(TaikoHitObject hitObject)
+            : base(hitObject)
+        {
+            Anchor = Anchor.CentreLeft;
+            Origin = Anchor.Centre;
+
+            RelativePositionAxes = Axes.X;
+
+            LifetimeStart = HitObject.StartTime - HitObject.PreEmpt * 2;
+            LifetimeEnd = HitObject.StartTime + HitObject.PreEmpt;
+
+            // Todo: Remove (suppresses Resharper)
+            AccentColour = Color4.White;
+
+            Children = new[]
+            {
+                CreateCircle()
+            };
+        }
+
+        protected override TaikoJudgementInfo CreateJudgementInfo() => new TaikoJudgementInfo();
+
+        protected override void UpdateState(ArmedState state)
+        {
+        }
+
+        /// <summary>
+        /// Sets the scroll position of the DrawableHitObject relative to the offset between
+        /// a time value and the HitObject's StartTime.
+        /// </summary>
+        /// <param name="time"></param>
+        protected void UpdateScrollPosition(double time)
+        {
+            MoveToX((float)((HitObject.StartTime - time) / HitObject.PreEmpt));
+        }
+
+        protected override void Update()
+        {
+            UpdateScrollPosition(Time.Current);
+        }
+
+        protected abstract ScrollingCirclePiece CreateCircle();
+    }
+}