How to add built-in shaders in an Android build

[dibenedetto]

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EDIT:
I found that putting them into the "Alwais Included Shaders" solves the problem.
Anyway, is this the right solution?

I recently found that a possible bug in using built-in shaders, in BuiltInMaterialGenerator.cs :

s_DefaultMaterial = GetPbrMetallicRoughnessMaterial();
s_DefaultMaterial.name = DefaultMaterialName;

If line 1 returns null we have a problem in line 2.

In my case, the app running on an Android device doesn't find the glTF/PbrMetallicRoughness shader, even if it is apparently included in glTFast package. If I try to copy the package Shader directory into project Assets, I get a conflict warning from the editor.

How will I ensure that those shaders are included in the Android build?