Guard against potential usage of DrawablePool without being added to hierarchy

[peppy]

This has caught me twice to date. It still works roughly correctly, but won't initialise the pool to the correct default size, taking away the startup performance benefits of pooling.

Breaking changes

DrawablePool.Get() wil now throw if the DrawablePool did not begin load

Previously the pool would return drawables correctly but it would not initialise the pool to correct default size, causing overheads on drawable retrieval (usually on update thread).

[bdach]

Changes to apply game-side after this (aside from the 2 pulls linked above):

diff --git a/osu.Game.Rulesets.Mania.Tests/Skinning/TestSceneBarLine.cs b/osu.Game.Rulesets.Mania.Tests/Skinning/TestSceneBarLine.cs
index ab9f57ecc3..8fd0327065 100644
--- a/osu.Game.Rulesets.Mania.Tests/Skinning/TestSceneBarLine.cs
+++ b/osu.Game.Rulesets.Mania.Tests/Skinning/TestSceneBarLine.cs
@@ -26,27 +26,32 @@ private void recreate(Func<IEnumerable<BarLine>>? createBarLines = null)
                 new StageDefinition(4),
             };
 
-            SetContents(_ => new ManiaPlayfield(stageDefinitions).With(s =>
+            SetContents(_ =>
             {
-                if (createBarLines != null)
+                var playfield = new ManiaPlayfield(stageDefinitions);
+                LoadComponent(playfield);
+                return playfield.With(s =>
                 {
-                    var barLines = createBarLines();
+                    if (createBarLines != null)
+                    {
+                        var barLines = createBarLines();
 
-                    foreach (var b in barLines)
-                        s.Add(b);
+                        foreach (var b in barLines)
+                            s.Add(b);
 
-                    return;
-                }
+                        return;
+                    }
 
-                for (int i = 0; i < 64; i++)
-                {
-                    s.Add(new BarLine
+                    for (int i = 0; i < 64; i++)
                     {
-                        StartTime = Time.Current + i * 500,
-                        Major = i % 4 == 0,
-                    });
-                }
-            }));
+                        s.Add(new BarLine
+                        {
+                            StartTime = Time.Current + i * 500,
+                            Major = i % 4 == 0,
+                        });
+                    }
+                });
+            });
         }
     }
 }
diff --git a/osu.Game.Rulesets.Osu.Tests/TestSceneDrawableJudgement.cs b/osu.Game.Rulesets.Osu.Tests/TestSceneDrawableJudgement.cs
index 874130233a..122d23c77f 100644
--- a/osu.Game.Rulesets.Osu.Tests/TestSceneDrawableJudgement.cs
+++ b/osu.Game.Rulesets.Osu.Tests/TestSceneDrawableJudgement.cs
@@ -69,7 +69,10 @@ private void showResult(HitResult result)
                     DrawablePool<TestDrawableOsuJudgement> pool;
 
                     if (poolIndex >= pools.Count)
+                    {
                         pools.Add(pool = new DrawablePool<TestDrawableOsuJudgement>(1));
+                        LoadComponent(pool);
+                    }
                     else
                     {
                         pool = pools[poolIndex];