Support rotation

In WP 5.5 the image block introduced inline image editing. We could potentially hijack the API endpoint that generates a modified image if we also supported rotation. Rotation does add a bit of complexity when calculating & applying crop dimensions however. We'll need to apply this consistently eg.

- Rotate & then crop (crop dimensions are for rotated image)
- Crop & then rotate (crop dimensions are pre-rotation)

The WP editor assumes rotating and then cropping, and that fits the model of values an image editor UI is likely to be able to produce so I feel it's the best approach.