Billboarding for Unity surface shaders

gistfile1.c

void vert(inout appdata_full v, out Input o) 
{
  UNITY_INITIALIZE_OUTPUT(Input, o);
  
  // get the camera basis vectors
  float3 forward = -normalize(UNITY_MATRIX_V._m20_m21_m22);
  float3 up = float3(0, 1, 0); //normalize(UNITY_MATRIX_V._m10_m11_m12);
  float3 right = normalize(UNITY_MATRIX_V._m00_m01_m02);
  
  // rotate to face camera
  float4x4 rotationMatrix = float4x4(right,   0,
                                     up,      0,
                                     forward, 0,
                                     0, 0, 0, 1);
  
  //float offset = _Object2World._m22 / 2;
  float offset = 0;
  v.vertex = mul(v.vertex + float4(0, offset, 0, 0), rotationMatrix) + float4(0, -offset, 0, 0);
  v.normal = mul(v.normal, rotationMatrix);
}