Create a glass material for the benchmark demo application Qt 6 version

Doing glass with PrincipledMaterial works very poorly, so there is a need to create a custom glass material to be used in the benchmark demo app, so it can be compared with 5.15 version, which has GlassMaterial in the material library.

This task can be done by creating a new example that provides a glass material created using CustomMaterial. That same material from the example can then be copied to the benchmark demo.