Bug: glTF import fails to set mMethod in aiMesh #5303

Describe the bug
After importing a glTF mesh with morphs, the mMethod field (in aiMesh) ought to be determined. However, it's typically aiMorphingMethod_UNKNOWN (0x0).

To Reproduce

1. Download a glTF model file with morph animation. There are at least 3 in the Khronos sample models, so pick one:
   • AnimatedMorphCube
   • AnimatedMorphSphere
   • MorphPrimitivesTest
2. Import the glTF model to an aiScene.
3. Iterate over the meshes in the aiScene; verify that all of them have mesh.mMethod == 0x0

Expected behavior
At least one of the meshes (specifically, all those with mNumAnimMeshes > 0) should have a determined morph method, either aiMorphingMethod_VERTEX_BLEND (0x1) or aiMorphingMethod_MORPH_NORMALIZED (0x2) or aiMorphingMethod_MORPH_RELATIVE (0x3).

If none of these methods accurately describes the morph method in the glTF spec, then another (non-zero) method should be defined in "mesh.h" .

Screenshots
not applicable

Platform (please complete the following information):

• OS: Mint Linux
• Browser: Firefox
• Version: the 59b1234 commit of Assimp) (2023-09-14)