OccluderInstance3D baking does not ignore ShaderMaterials that are transparent

### Tested versions

4.2.2-stable

### System information

Windows 10 (I don't think this matters at all)

### Issue description

According to @Calinou, the OccluderInstance3D only checks for if a SurfaceMaterial has the transparency flag. Instead it should read the cull mode defined in the Shader, which should be even easier to read, according to him.

This should also be applied to the Lightmapper.

CC @Calinou @clayjohn

### Steps to reproduce

Create a MeshInstance3D with the shader included as the MRP as the material. Observe as it fails to be ignored by the OccluderInstance3D

### Minimal reproduction project (MRP)

[glass.zip](https://github.com/godotengine/godot/files/15069029/glass.zip)