Point Cloud attenuation/EDL + custom post processing stage : WEBGL error :cannot use both gl_FragData and gl_FragColor

When a custom post processing stage based on "highlight" sandcastle example is applied, if a point cloud 3D tile **with an active pointcloudshading attenuation** is loaded, a WEBGL issue is raised :

_[Cesium WebGL] Fragment shader compile log: ERROR: cannot use both gl_FragData and gl_FragColor_

Release: 1.52
OS : Windows8.1 / Chrome 71

The sandcastle example to reproduce it.
https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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

**Quick analysis (To be confirmed by CESIUM Team)**
EDL is switching from gl_FragColor to gl_FragData[0] and updates shader source stack but when it is done, the pick shader is not in the list, it is added after.

void czm_log_depth_main()
{
czm_point_cloud_post_process_main();
**gl_FragData[1]** = czm_packDepth(gl_FragCoord.z);
}
#line 0
#ifdef GL_EXT_frag_depth
#endif
void czm_non_pick_main()
{
czm_log_depth_main();
czm_writeLogDepth();
}
#line 0
void main()
{
czm_non_pick_main();
if (gl_FragColor.a == 0.0) {
discard;
}
**gl_FragColor** = czm_pickColor;
}

**Attempt to quick fix it**
Setting a #define (see below) makes successful the compilation but it introduces a **border effect**:
_The point cloud with attenuation is flicking, only briefly visible at update or during view transition if :_

- requestRenderMode: true
- maximumRenderTimeChange: 2

So potentially not the best way to solve it.

#define PointCloudEDL
void main()
{
czm_point_cloud_post_process_main();
gl_FragData[1] = czm_packDepth(gl_FragCoord.z);
}

void main()
{
czm_non_pick_main();
#ifdef PointCloudEDL
if (gl_FragData[0].a == 0.0) {
discard;
}
gl_FragData[0] = ...
#else
if (gl_FragColor.a == 0.0) {
discard;
}
gl_FragColor = ....
#endif
}