Masking to make a health bar

Here is how I ended up making my health bar. It uses an image of a gradient from white to transparent for the mask.

Here is the shader I wrote for the bar. Use the texture of the health bar for Base and the gradient for Mask.

 Shader "Custom/Mask" {
     Properties {
         _Color ("Main Color", Color) = (1,1,1,1)
         _MainTex ("Base (RGBA)", 2D) = "white" {}
         _MaskTex ("Mask (RGBA)", 2D) = "white" {}
     }
     SubShader {
         Tags { "Queue" = "Transparent" }
         Pass {
             Blend SrcAlpha OneMinusSrcAlpha
             SetTexture [_MainTex] {
                 constantColor [_Color]
                 combine Constant * Texture
             }
             SetTexture [_MaskTex] {
                 combine Previous, Previous * Texture
             }
         }
     }
     Fallback "Sprites/Default"
 }

Here is the script attached to the bar. At percent = 0, the bar is invisible, and percent = 1, the bar is completely unveiled.

 using UnityEngine;
 using System.Collections;
 
 public class HealthBar : MonoBehaviour
 {
     [Range(0,1)]
     public float percent;
     float oldPercent = 0;

     void Update ()
     {
         if (oldPercent != percent) {
             renderer.material.SetTextureOffset ("_MaskTex", Vector2.right * (1 - percent * 2));
             oldPercent = percent;
         }
     }
 }
 

This took me forever to figure out but it works great. I like how I can lerp the health since I don't use discrete frames. One thing I don't understand is this warning: "Material uses fixed function shader. It is not compatible with SpriteRenderer"