Fix DSPFC not considerng parent padding correctly

osu.Framework.Tests/Visual/Containers/TestSceneDrawSizePreservingFillContainer.cs

···
 // Copyright (c) ppy Pty Ltd <[e-mail]>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using NUnit.Framework;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Graphics.Shapes;
+using osu.Framework.Utils;
 using osuTK;
 using osuTK.Graphics;
 
···
         public TestSceneDrawSizePreservingFillContainer()
         {
             DrawSizePreservingFillContainer fillContainer;
+
             Child = new Container
             {
                 Anchor = Anchor.Centre,
···
             AddSliderStep("Height", 50, 650, 500, v => Child.Height = v);
 
             AddStep("Override Size to 1x1", () => Child.Size = Vector2.One);
+        }
+
+        [Test]
+        public void TestParentPadding()
+        {
+            Box fullBox = null;
+            Box innerBox = null;
+
+            AddStep("create container", () =>
+            {
+                Child = new Container
+                {
+                    Anchor = Anchor.Centre,
+                    Origin = Anchor.Centre,
+                    Size = new Vector2(512, 384),
+                    Children = new Drawable[]
+                    {
+                        fullBox = new Box
+                        {
+                            RelativeSizeAxes = Axes.Both,
+                            Colour = Color4.Red,
+                        },
+                        new Container
+                        {
+                            RelativeSizeAxes = Axes.Both,
+                            Padding = new MarginPadding { Left = 100 },
+                            Children = new Drawable[]
+                            {
+                                new DrawSizePreservingFillContainer
+                                {
+                                    Child = innerBox = new Box
+                                    {
+                                        Size = new Vector2(1024, 768),
+                                        Colour = Color4.Pink,
+                                        Alpha = 0.3f
+                                    }
+                                },
+                            }
+                        },
+                    }
+                };
+            });
+
+            AddAssert("inner box stops at edge of right box", () => Precision.AlmostEquals(fullBox.ScreenSpaceDrawQuad.TopRight, innerBox.ScreenSpaceDrawQuad.TopRight));
         }
     }
 }

osu.Framework/Graphics/Containers/DrawSizePreservingFillContainer.cs

···
         {
             base.Update();
 
-            Vector2 drawSizeRatio = Vector2.Divide(Parent.DrawSize, TargetDrawSize);
+            Vector2 drawSizeRatio = Vector2.Divide(Parent.ChildSize, TargetDrawSize);
 
             switch (Strategy)
             {