Mesh's buffer size does not update properly

[stlemme]

When updating the buffers of a mesh (i.e. position, normal, texcoord, etc.) and their length changes then the content is updated with the wrong buffer size. This affects meshes as well as assetmeshes.

An example showcasing this issue is available at http://stlemme.github.io/xml3d-experimental/buffersize-bug/terrain-stitching-test.html

When updating the index buffer of a mesh only, it does work properly.