Mask a HP Bar

I'm trying to make an HP Bar and found 2 answers

here http://answers.unity3d.com/questions/7846/how-do-i-make-a-progress-bar.html

and here http://answers.unity3d.com/questions/15774/scalemodescaleandcrop-crop-direction.html

that helped me, but it's not exactly what I want because my texture is not a rectangle, it is a parallelogram, like this,

So when I change the width of the BeginGroup, since it uses a rectangle, the texture border becomes vertical. Is there a way to mask the texture but keeping the border like a parallelogram?

This is the code I'm using (the variable redBarWidth is modiffied when the character receives damage or gains hp)

 public void OnGUI()
 {
     GUI.BeginGroup(new Rect(redBarX, redBarY, redBarWidth, redBar.height));
     GUI.DrawTexture(new Rect(0, 0, redBar.width, redBar.height), redBar, ScaleMode.StretchToFill);
     GUI.EndGroup();
 }