Added read support for image blobs

[yannickulrich]

This adds support for the native image insertions in notebooks on 3.27 or later. I've opened similar PRs with the python reader (ricklupton/rmscene#52 and ricklupton/rmc#50). The construction is based on remarkable-shapes which already has this ability in ruby.

On the top-level, images are defined in a ImageEntry (block type 0x0E) via a UUID. They can have a flag which currently always evaluates to [17, 0]. They are then references in the tree as a Image (block type 0x0F, item type 0x07). This uses the UUID defined earlier and includes image placement as a list of image vertices (x, y, u, v tuples)

[
  left, top, 0.0, 0.0,
  right, top, 1.0, 0.0,
  right, bottom, 1.0, 1.0,
  left, bottom, 0.0, 1.0
]

Exporting images is (in principle) pretty easy

func drawImage(image *parser.Image, w io.Writer, indent string) {
	fmt.Fprintf(w, "%s<g transform=\"translate(%.3f, %.3f)\">\n",
		indent, scale(float64(image.X)), scale(float64(image.Y)))
	fmt.Fprintf(w, "%s\t<image height=\"%.3f\" width=\"%.3f\" href=\"%s\"/>\n",
		indent, scale(float64(image.Height)), scale(float64(image.Width)), image.Filename.Value)
	fmt.Fprintf(w, "%s</g>\n", indent)
}

However, there is a question of how to load the image in the first place. An rmdoc file looks like

├── acf9e956-3646-4c6f-bf4a-c09ffdd17d77.content
├── acf9e956-3646-4c6f-bf4a-c09ffdd17d77.metadata
├── acf9e956-3646-4c6f-bf4a-c09ffdd17d77
│   ├── 34d0c003-1ff8-40a3-bba2-3b7807a3632c.png
│   └── f25ec2ea-bfb7-4777-ae8c-7c6711fa084c.png
└── acf9e956-3646-4c6f-bf4a-c09ffdd17d77.rm

Since the reader is just given a file handle for the .rm file, it won't be able to figure out how to load the .png files next to it. If you have a suggestion on how to best handle this, please let me know!