[XNA] Fixed Alpha in SkeletonRenderer.cs

Alpha was not displaying correctly due to changes in XNA 4.0, to use Alphablend (BlendState SkeletonRenderer is set to use) you must set the alpha of a colour by multiplying a colour by a float value of 0f to 1f. (eg: new Color(255,255,255)*0.5f will result in a half transparent white colour)