Visual artifacts with material overlay in compatibility mode

[uguu-org]

Surfaces with MToon shader appears to exhibit visual artifacts with overlay material when rendered in compatibility mode:

Maybe some kind of z-fighting due to precision issues? All these artifacts go away if I do a global replacement of VIEW_MATRIX * MODEL_MATRIX with MODELVIEW_MATRIX under addons/Godot-MToon-Shader/*.gdshader (21 instances across 7 files):

This problem doesn't happen with Forward+ renderer.

Unfortunately, I can't seem to reproduce this bug on any other model, but the steps I took were:

0. Set compatibility mode.
1. Import VRM with BasisU disabled (see Importing VRM causes "Basis universal cannot unpack level 1" errors #58).
2. Select body node, then add new material under "Geometry -> Material Overlay".
3. Set transparency to "Alpha" and Cull mode to "Disabled".

I understand this is not the best bug report without a proper test case, but Godot's documentation also says "use if possible" next to MODELVIEW_MATRIX, so doing this replacement seems like a good idea.

[lyuma]

I have applied your suggestion in master. Thanks!

I wonder if Godot 3.x has the same issue. I will consider a backport if I update VRM/MToon for Godot 3