Textures specified in a Decal are not detected as used in 3D

[Calinou]

Related to #62032.

Godot version

4.0.alpha (1847094)

System information

Fedora 36, GeForce GTX 1080 (NVIDIA 510.68.02)

Issue description

Unlike textures specified in a BaseMaterial3D, textures specified in a Decal are not detected as used in 3D. This causes 3 issues of varying importance:

• VRAM compression is not used, which wastes memory for high-resolution textures.
• Normal map compression (RGTC) is not used, which wastes memory for textures that could use VRAM compression.
• The roughness filter on import is not used, which increases the amount of visible specular aliasing.

Steps to reproduce

• Add a MeshInstance3D node with a BoxMesh (to see the Decal on top).
• Add a Decal node.
• Specify four different textures in its Albedo, Normal, ORM and Emission properties.
• Notice how textures are never detected as used in 3D, and are never reimported with VRAM compression enabled.

Minimal reproduction project

test_decal_detect_3d.zip