Watermark example for map attribution and logo

[chrisgervang]

Demonstrate rendering attribution text and an image logo using an orthographic view along-side a map view.

This feature was first demonstrated in #204 for data-driven widgets. Building this simpler watermark use case seems like a good place to start, and can be used in browser tests.

Builds on visgl/deck.gl#6329, and MapboxOverlay Multi-view usage.