Question PBR shader with aditional separate texture mask

Hello,

I have a problem with a shader which I can't solve.
In this shader I want to use a render texture from a camera as albedo or base color including its transparency.
Additionally I want to mask this texture with a separate black & white mask so it is only projected on the parts of the model where the mask allows it.

I am no shader programmer so I'm afraid can't solve this by myself.
Ideally it should work as the standard pbr or the autodesk interactive shader with the additional mask.

The shader I have so far looks like this but it does not work:

Code (CSharp):

1. // Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'
2.  
3. Shader "Custom/PrintMask"
4. {
5.     Properties {
6.         _Color ("Color", Color) = (1,1,1,1)
7.         [NoScaleOffset] _MainTex ("Albedo (RGB)", 2D) = "white" {}
8.         [Toggle] _UseMetallicMap ("Use Metallic Map", Float) = 0.0
9.         [NoScaleOffset] _MetallicGlossMap("Metallic", 2D) = "black" {}
10.         [Gamma] _Metallic ("Metallic", Range(0,1)) = 0.0
11.         _Glossiness ("Smoothness", Range(0,1)) = 0.5
12.         _BumpScale("Scale", Float) = 1.0
13.         [NoScaleOffset] _BumpMap("Normal Map", 2D) = "bump" {}
14.         _Mask ("Culling Mask", 2D) = "white" {}
15.         _Cutoff("Alpha Cutoff", Range(0.01,1)) = 0.5
16.     }
17.  
18.     SubShader {
19.  
20.         Tags { "Queue"="AlphaTest" "IgnoreProjector"="True" "RenderType"="TransparentCutout" }
21.         Blend SrcAlpha OneMinusSrcAlpha
22.         LOD 200
23.         ZWrite Off
24.         Cull Off
25.  
26.         Pass { // Prepass depth write
27.             ColorMask 0
28.             ZWrite On
29.             CGPROGRAM
30.             #pragma vertex vert
31.             #pragma fragment frag
32.  
33.             #include "UnityCG.cginc"
34.             struct v2f {
35.                 float4 vertex : SV_POSITION;
36.                 float2 texcoord : TEXCOORD0;
37.             };
38.             sampler2D _MainTex;
39.             fixed _Cutoff;
40.             v2f vert (appdata_img v)
41.             {
42.                 v2f o;
43.                 o.vertex = UnityObjectToClipPos(v.vertex);
44.                 o.texcoord = v.texcoord;
45.                 return o;
46.             }
47.             fixed4 frag (v2f i) : SV_Target
48.             {
49.                 fixed4 col = tex2D(_MainTex, i.texcoord);
50.                 clip(col.a - _Cutoff);
51.                 return 0;
52.             }
53.             ENDCG
54.         }
55.  
56.         Pass {
57.             Tags {"LightMode"="ShadowCaster"}
58.             ZWrite On
59.             Cull Off
60.             CGPROGRAM
61.             #pragma vertex vert
62.             #pragma fragment frag
63.             #pragma multi_compile_shadowcaster
64.             #include "UnityCG.cginc"
65.             struct v2f {
66.                 V2F_SHADOW_CASTER;
67.                 float2 texcoord : TEXCOORD1;
68.             };
69.             v2f vert(appdata_base v)
70.             {
71.                 v2f o;
72.                 TRANSFER_SHADOW_CASTER_NORMALOFFSET(o)
73.                 o.texcoord = v.texcoord;
74.                 return o;
75.             }
76.  
77.             sampler2D _MainTex;
78.             fixed _Cutoff;
79.             float4 frag(v2f i) : SV_Target
80.             {
81.                 fixed4 col = tex2D(_MainTex, i.texcoord);
82.                 clip(col.a - _Cutoff);
83.                 SHADOW_CASTER_FRAGMENT(i)
84.             }
85.             ENDCG
86.         }
87.  
88.         CGPROGRAM
89.         #pragma surface surf Standard fullforwardshadows alpha:fade nolightmap
90.         #pragma shader_feature _USEMETALLICMAP_ON
91.         #pragma target 3.0
92.         sampler2D _MainTex;
93.         sampler2D _MetallicGlossMap;
94.         sampler2D _BumpMap;
95.         struct Input {
96.             float2 uv_MainTex;
97.             fixed facing : VFACE;
98.         };
99.         half _Glossiness;
100.         half _Metallic;
101.         fixed4 _Color;
102.         half _BumpScale;
103.         fixed _Cutoff;
104.         void surf (Input IN, inout SurfaceOutputStandard o) {
105.             // Albedo comes from a texture tinted by color
106.             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
107.             o.Albedo = c.rgb;
108.             #ifdef _USEMETALLICMAP_ON
109.             fixed4 mg = tex2D(_MetallicGlossMap, IN.uv_MainTex);
110.             o.Metallic = mg.r;
111.             o.Smoothness = mg.a;
112.             #else
113.             o.Metallic = _Metallic;
114.             o.Smoothness = _Glossiness;
115.             #endif
116.             // Rescales the alpha on the blended pass
117.             o.Alpha = saturate(c.a / _Cutoff);
118.             o.Normal = UnpackScaleNormal(tex2D(_BumpMap, IN.uv_MainTex), _BumpScale);
119.             o.Normal.z *= IN.facing;
120.         }
121.         ENDCG
122.  
123.         Blend SrcAlpha OneMinusSrcAlpha
124.         Pass
125.         {
126.           SetTexture [_Mask] {combine texture}
127.           SetTexture [_MainTex] {combine texture, previous}
128.         }
129.     }
130.  
131.     FallBack "Diffuse"
132. }
133.  

I hope someone can help me with this problem.