This fixes https://github.com/godotengine/godot/issues/20377 , where blend shape scales the mesh if the mesh is also skinned.
The issue was that the blend shape was trying to blend using BLEND_SHAPE_MODE_RELATIVE (directly adding everything in morph shape as displacement), while bone weights were copied in the morph shape, which resulted in 2x bone weights causing mesh to become bigger when blended.
Setting the blend mode to BLEND_SHAPE_MODE_NORMALIZED while guaranteeing the data is correct fixes the issue (previously treating gltf2's morph displacement data as blend target data). Ideally we still want to use BLEND_SHAPE_MODE_RELATIVE since it may need much less data, but that seems to require a larger refactor?
d9d6119b6e