Add Bindable<T>.CopyTo and use in GetUnboundCopy implementation

[peppy]

This method is used heavily in AudioAdjustments, which is how I got here.

osu-framework/osu.Framework/Audio/AudioAdjustments.cs

Line 68 in ba13853

var aggregate = getAggregate(type) = new AggregateBindable<double>(getAggregateFunction(type), getProperty(type).GetUnboundCopy());

Method	Mean	Error	StdDev	Gen 0	Allocated
GetUnboundCopyOld	940.6 ns	6.14 ns	5.13 ns	0.0629	736 B
GetUnboundCopyNew	38.95 ns	0.848 ns	1.160 ns	0.0153	176 B

Breaking Changes

Bindable<T>.CopyTo should be implemented for any custom bindables

To improve the performance of creating clones of bindables (ie. via GetUnboundCopy()), the copy portion of BindTo() has been split out into its own method. If you have any custom bindable types which were copying values across within a BindTo() override, please move the copy operation to CopyTo() instead. Pay special attention to the direction of assignment, which will reverse from what it was in BindTo.

A fix should look something like this:

diff --git a/osu.Framework/Bindables/BindableNumber.cs b/osu.Framework/Bindables/BindableNumber.cs
index 6d605667f..430da79d0 100644
--- a/osu.Framework/Bindables/BindableNumber.cs
+++ b/osu.Framework/Bindables/BindableNumber.cs
@@ -199,12 +199,12 @@ protected void TriggerPrecisionChange(BindableNumber<T> source = null, bool prop
                 PrecisionChanged?.Invoke(precision);
         }
 
-        public override void BindTo(Bindable<T> them)
+        public override void CopyTo(Bindable<T> them)
         {
             if (them is BindableNumber<T> other)
-                Precision = other.Precision;
+                other.Precision = Precision;
 
-            base.BindTo(them);
+            base.CopyTo(them);
         }
 
         public override void UnbindEvents()

[peppy]

osu! side changes required: peppy/osu@1632df1

Seems to work okay.