Support FutureOn texture mapping

[pmconne]

Is your feature request related to a problem? Please describe.
FutureOn uses a technique for pattern mapping that generates UV coords in the fragment shader to tile the texture and blend it, interpolating between two levels of detail. iTwin.js currently does not support this mode.

Describe the solution you'd like
Implement support for this mapping mode.

Describe alternatives you've considered
Continue to use UVs pregenerated by the FutureOn connector. This produces a less desirable result. It also prevents us from using different UV coords for normal mapping.

Additional context
The technique is described here.

[markschlosseratbentley]

On track for 4.0. (Still need to come up with a name).

[markschlosseratbentley]

Still on track.

[markschlosseratbentley]

Plan for a PR this week.

[markschlosseratbentley]

Currently working the backend -- ready to make PRs for these things.