public void Draw(List<ITouchPointSizeColor> touchPoints)
{
/*
* We use NonPremultiplied when drawing our picture - this allows the user to reduce the alpha value (transparency)
* and then build up the color by drawing over the top of it.
* Also if painting one color over another then they will merge slightly - e.g. painting a red over a green will make a yellow.
* Other alternatives are Additive, Opaque and AlphaBlending.
* Additive will eventually turn everything white the more you colour it.
* Opaque simply ignores the alpha value - hence useful when drawing controls as we want to completely replace the previous image
* AlphaBlending and NonPremultiplied are very similar - NonPremultiplied uses Blend.SourceAlpha when mixing the source and
* destination colours - BlendAlpha uses BlendOne. (See Monogame/BlendState.cs)
*
*/
this.graphicsDisplay.BeginRenderNonPremultiplied();
this.DrawPicture(touchPoints);
this.graphicsDisplay.EndRender();
}