Only allow one drag event at a time

Currently, when a game specified that drags can be initiated by more than just left mouse via..

osu-framework/osu.Framework/Input/MouseButtonEventManager.cs

Lines 24 to 27 in 36afef9

    
           /// <summary> 
        
           /// Whether dragging is handled by the managed button. 
        
           /// </summary> 
        
           public abstract bool EnableDrag { get; }

.. one OnDragStart event triggers per mouse button. I'd argue that this shouldn't be the case, and once a drag operation is started no more can trigger until that operation ends.

The current behaviour can lead to oversights as an average framework consumer would not expect multiple concurrent drag operations to be a thing.

This caused the failure here: ppy/osu#26353

	/// <summary>
	/// Whether dragging is handled by the managed button.
	/// </summary>
	public abstract bool EnableDrag { get; }