ImageMaskTransition has problem

[zengzhining]

I use ImageMaskTransition but the background color is not black.

I think the problem is in PreRender method, it should add Clear method? I add it and it works.

public override void PreRender(Batcher batcher)
{
    Core.GraphicsDevice.SetRenderTarget(_maskRenderTarget);
    // Core.GraphicsDevice.Clear( Color.Black );
    batcher.Begin(BlendState.AlphaBlend, Core.DefaultSamplerState, DepthStencilState.None, null);
    batcher.Draw(_maskTexture, _maskPosition, null, Color.White, _renderRotation, _maskOrigin,
        _renderScale, SpriteEffects.None, 0);
    batcher.End();
    Core.GraphicsDevice.SetRenderTarget(null);
}