fix: alpha does not work properly on Android

Author: homuler

Packages/com.github.homuler.mediapipe/PackageResources/Shaders/MaskShader.shader

@@ -12,6 +12,8 @@ Shader "Unlit/MediaPipe/Mask Shader"
     SubShader
     {
         Tags { "RenderType"="Transparent" }
+        ZWrite Off
+        Blend SrcAlpha OneMinusSrcAlpha
         LOD 100
 
         Pass

Packages/com.github.homuler.mediapipe/PackageResources/Shaders/OverlayMask.shader

@@ -65,7 +65,7 @@ Shader "Unlit/MediaPipe/Overlay Mask Shader"
                 fixed4 maskCol = tex2D(_MaskTex, i.uv);
                 int idx = int(i.uv.y * _Height) * _Width + int(i.uv.x * _Width);
                 float mask = _MaskBuffer[idx];
-                maskCol.a = mask;
+                maskCol.a = lerp(0.0, mask, step(_Threshold, mask));
                 fixed4 col = lerp(emptyCol, maskCol, step(_Threshold, mask));
 
                 // apply fog