Fix a couple of issues with density volume texture mask.

com.unity.render-pipelines.high-definition/HDRP/Lighting/Volumetrics/VolumeTextureAtlas.cs

         {
             if (textures.Contains(volumeTexture))
             {
-                textures.Add(volumeTexture);
+                textures.Remove(volumeTexture);
 
                 needTextureUpdate = true;
             }

com.unity.render-pipelines.high-definition/HDRP/Lighting/Volumetrics/VolumeVoxelization.compute

 
                 // Express the voxel center in the local coordinate system of the box.
                 float3 voxelCenterBS = mul(voxelCenterWS - obb.center, transpose(obbFrame));
-                float3 voxelCenterUV = (voxelCenterBS / obbExtents) * 0.5 + 0.5;
+                float3 voxelCenterUV = (voxelCenterBS / obbExtents);
 
             #if SOFT_VOXELIZATION
                 // We need to determine which is the face closest to 'voxelCenterBS'.

com.unity.render-pipelines.high-definition/HDRP/Lighting/Volumetrics/VolumetricLighting.cs

 
                 if (volumeAtlas != null)
                 {
-                    volumeAtlasDimensions.x = volumeAtlas.width / volumeAtlas.depth; // 1 / number of textures
+                    volumeAtlasDimensions.x = (float)volumeAtlas.width / volumeAtlas.depth; // 1 / number of textures
                     volumeAtlasDimensions.y = 1.0f / volumeAtlas.width;
                     volumeAtlasDimensions.z = volumeAtlas.width;
                 }