Make fallback bool into a function

Allows correct handling now that beatmap skins are also a thing.

osu.Game.Rulesets.Osu/Objects/Drawables/Pieces/ExplodePiece.cs

@@ -25,7 +25,7 @@ public ExplodePiece()
                 Blending = BlendingMode.Additive,
                 RelativeSizeAxes = Axes.Both,
                 Alpha = 0.2f,
-            }, false);
+            }, s => s.GetTexture("Play/osu/hitcircle") == null);
         }
     }
 }

osu.Game.Rulesets.Osu/Objects/Drawables/Pieces/FlashPiece.cs

@@ -29,7 +29,7 @@ public FlashPiece()
                 {
                     RelativeSizeAxes = Axes.Both
                 }
-            }, false);
+            }, s => s.GetTexture("Play/osu/hitcircle") == null);
         }
     }
 }

osu.Game.Rulesets.Osu/Objects/Drawables/Pieces/GlowPiece.cs

@@ -29,7 +29,7 @@ private void load(TextureStore textures)
                 Texture = textures.Get(name),
                 Blending = BlendingMode.Additive,
                 Alpha = 0.5f
-            }, false);
+            }, s => s.GetTexture("Play/osu/hitcircle") == null);
         }
     }
 }

osu.Game.Rulesets.Osu/Objects/Drawables/Pieces/NumberPiece.cs

@@ -40,7 +40,7 @@ public NumberPiece()
                         Colour = Color4.White.Opacity(0.5f),
                     },
                     Child = new Box()
-                }, false),
+                }, s => s.GetTexture("Play/osu/hitcircle") == null),
                 number = new OsuSpriteText
                 {
                     Text = @"1",

osu.Game/Skinning/SkinReloadableDrawable.cs

@@ -1,6 +1,7 @@
 // Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
 // Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
 
+using System;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics.Containers;
 
@@ -11,20 +12,21 @@ namespace osu.Game.Skinning
     /// </summary>
     public abstract class SkinReloadableDrawable : CompositeDrawable
     {
+        private readonly Func<ISkinSource, bool> allowFallback;
         private ISkinSource skin;
 
         /// <summary>
         /// Whether fallback to default skin should be allowed if the custom skin is missing this resource.
         /// </summary>
-        private readonly bool allowDefaultFallback;
+        private bool allowDefaultFallback => allowFallback == null || allowFallback.Invoke(skin);
 
         /// <summary>
         /// Create a new <see cref="SkinReloadableDrawable"/>
         /// </summary>
         /// <param name="fallback">Whether fallback to default skin should be allowed if the custom skin is missing this resource.</param>
-        protected SkinReloadableDrawable(bool fallback = true)
+        protected SkinReloadableDrawable(Func<ISkinSource, bool> allowFallback = null)
         {
-            allowDefaultFallback = fallback;
+            this.allowFallback = allowFallback;
         }
 
         [BackgroundDependencyLoader]

osu.Game/Skinning/SkinnableDrawable.cs

@@ -9,8 +9,8 @@ namespace osu.Game.Skinning
 {
     public class SkinnableDrawable : SkinnableDrawable<Drawable>
     {
-        public SkinnableDrawable(string name, Func<string, Drawable> defaultImplementation, bool fallback = true, bool restrictSize = true)
-            : base(name, defaultImplementation, fallback, restrictSize)
+        public SkinnableDrawable(string name, Func<string, Drawable> defaultImplementation, Func<ISkinSource, bool> allowFallback = null, bool restrictSize = true)
+            : base(name, defaultImplementation, allowFallback, restrictSize)
         {
         }
     }
@@ -31,7 +31,7 @@ public class SkinnableDrawable<T> : SkinReloadableDrawable
         /// <param name="defaultImplementation">A function to create the default skin implementation of this element.</param>
         /// <param name="fallback">Whther to fallback to the default implementation when a custom skin is specified but not implementation is present.</param>
         /// <param name="restrictSize">Whether a user-skin drawable should be limited to the size of our parent.</param>
-        public SkinnableDrawable(string name, Func<string, T> defaultImplementation, bool fallback = true, bool restrictSize = true) : base(fallback)
+        public SkinnableDrawable(string name, Func<string, T> defaultImplementation, Func<ISkinSource, bool> allowFallback = null, bool restrictSize = true) : base(allowFallback)
         {
             componentName = name;
             createDefault = defaultImplementation;