Support image flip

[rsimon]

See original Annotorious issue here: annotorious/annotorious#302

[rsimon]

Note that, in addition to just being able to flip the image in the viewer, we'd also need:

• Viewer flip in the "Auto Transcribe" preview dialog
• Support for sending flipped images to transcription services:
  • Leveraging IIIF services where possible
  • Doing in-memory image flipping otherwise