Align the triangle mesh implementation of physx and bullet backends

[yiwenxue]

The current triangle mesh implementations of different backends have the following differences:

1. The triangle mesh of the physx backend will calculate the center of mass, and the triangle mesh of the bullet backend will not
2. The physx backend only uses the first submesh, and the bullet backend will use all submesh

It seems that the triangle mesh implementation of the physx backend is more in line with expectations, so it is recommended that the triangle mesh of the bullet backend also be aligned to the physx backend.

mesh attach point
bullet mesh collider

[yiwenxue]

• [ ] physics engine froze when enabling the convex option in the mesh collider for some mesh.