Add C++ equivalents for `from_rgb{a}8{_premultiplied}` to `sixtyfps::Image`

[tronical]

The Rust API for sixtyfps::Image allows for the creation of image objects from pixel buffers. We need API offering the same functionality for the C++ API.

[ogoffart]

We also need to have good documentation, for both Rust and C++ on how to generate image integrated with a sixtyfps ui

[DaMilyutin]

Waiting eagerly! 😃
Can you provide some estimation on when this feature will be implemented?