_MatcapColor was added to the MToon shader, which multiplies against the color from _SphereAdd, without adding any way for the VRM 0.0 loader to set it to something other than the default value of vec4(0.0, 0.0, 0.0, 1.0), meaning that it would always have zero additive influence.
This commit also flips the vertical axis of matcapUv in the MToon shader, because the _SphereAdd texture was showing up inverted in Godot.
Breaking commit: https://github.com/V-Sekai/godot-vrm/commit/9aa991f3dae34c5231c121be7f57de5b7e10d77f
_MatcapColor was added to the MToon shader, which multiplies against the color from _SphereAdd, without adding any way for the VRM 0.0 loader to set it to something other than the default value of vec4(0.0, 0.0, 0.0, 1.0), meaning that it would always have zero additive influence.
This commit also flips the vertical axis of matcapUv in the MToon shader, because the _SphereAdd texture was showing up inverted in Godot.