OpenGL: Mesh/Material resource preview thumbnails no longer render

[Calinou]

Tested versions

• Reproducible in: 4.3.dev3, 4.3.beta da4f6e4
• Not reproducible in: 4.2.2.stable, 4.3.dev2

System information

Godot v4.3.beta (da4f6e4) - Fedora Linux 40 (KDE Plasma) - X11 - GLES3 (Compatibility) - NVIDIA GeForce RTX 4090 (nvidia; 555.58.02) - 13th Gen Intel(R) Core(TM) i9-13900K (32 Threads)

Issue description

Since 4.3.dev3 (displays fallback text) and 4.3.dev4 (displays blank space), resource previews for Mesh and Material are broken when using the Compatibility rendering method:

Forward+ (working)	Compatibility (broken)

Other previews such as Texture and Gradient still work.

I bisected the regression to e5454cd (first commit where it shows fallback text instead of the correct preview). cc @RandomShaper

Steps to reproduce

• Create a MeshInstance3D node in a project that uses the Compatibility rendering method.
• Assign any PrimitiveMesh resource to it.
• Assign a new StandardMaterial3D in Surface Material Override.
• Notice how no thumbnails appear.

Minimal reproduction project (MRP)

test_mesh_material_preview.zip

Select the MeshInstance3D node in node_3d.tscn to see the broken thumbnails.