Audit and harden image decoding stuff against huge inputs (DoS) (#1110)

* Image support: add bmp/qoi,ico, webp, SVG in `Image` widget, 16-bit png

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.

* Audit and harden image decoding stuff against huge inputs (DoS)

Bound the size of the decoded image, pixel count, frame counts (for animated),
range of SVG sniffing, and encoded file size.
Only once we run those checks do we actually alloc a buffer for the decoded image. before allocating decode buffers. Validate

Add various other checks within the vendored image decoding libraries too.