Image support: add bmp/qoi,ico, webp, SVG in `Image` widget, 16-bit png (#1108)

Generally, this commit makes improvements to image decoding and rendering.

Added a bunch of functions for image discovery / metadata gathering:
`decode_image_from_data()`, `image_size_by_data()`, `looks_like_svg()`

Added more `Image[Ref]` functions for other image formats:
`ImageRef::load_{bmp,qoi,ico,gif,webp,svg}_from_data()`, plus a nice
convenience fn for auto-detec+load: `load_image_from_data()`.

Added cheap, lazily-init'd support for SVGs within the `Image` widget.

Fixed some issues with aspect ratio being clobbered during image rotation.