Geometry issues in BoomBoxWithAxes

[lexaknyazev]

• Plus signs have holes in the corners.
• Letters have strange irregularities on the flat surfaces.
• The scene size is too small considering the glTF units convention.

/cc @bghgary

[DRx3D]

Unless there is a PR to Sample Assets by 27 Nov, this issue will be transferred to that repo for handling.