Send image dimensions in m.image event

[turt2live]

Currently nheko doesn't send the image dimensions, which can cause other clients (like riot) to jump around as it loads the image. Although this is a bug for the other clients, it shouldn't take much to send the dimensions alongside the image.

For reference, here's an event body that is "good enough" for most clients:

  "content": {
    "body": "FILENAME.jpg",
    "info": {
      "mimetype": "image/jpeg",
      "h": 1080,
      "w": 1920,
      "size": 86412
    },
    "msgtype": "m.image",
    "url": "mxc://matrix.org/some_media_id"
  },