Identify useful functionality to add to the public API

[FreezyLemon]

The current API surface is fairly small, and there are some common patterns in downstream code that might make sense to upstream. Some ideas:

• Resolution (WxH) wrapper, especially for subsampling concerns (lot of manual x >> xdec etc.)
• Regions (borrows over particular rectangles of a plane), see for example PlaneOffset and PlaneRegion in av-scenechange (rav1e also similar implementations)

Feel free to comment more if you can think of any, or discuss my suggestions :)

[FreezyLemon]

From rust-av/yuvxyb#64 : Technically, everything's "there" to use uninitialized planes and create a frame, but the resulting code is not very ergonomic and we miss out on the FrameBuilder::build validation. There's probably some better API to be made here.

[FreezyLemon]

Pixel: Into<f32>?