Multi-window + camera UI example

[ickshonpe]

What problem does this solve or what need does it fill?

There are only a few examples: multiple_windows, split_screen and render_ui_to_texture that use anything other than a single camera, single window setup and split_screen just has some text and some buttons, the other two just have text. If a change breaks the box shadow implementation for multi-window UI none our current examples or tests will capture this.

What solution would you like?

An example using multiple windows and cameras displaying different UI layouts to each window that demonstrate that all the different UI features like shadows, materials, overflow, interactions, fonts, texture slicing etc, still work with mult-window UI.