The cairo image mime types, eg CAIRO_MIME_TYPE_JPEG, allow
applications to attached the original encoded image file. This allows
cairo backends that support the image encoding to embed the encoded
data directly in the ouptut instead of recompressing the image bitmap
with Flate.

The CAIRO_MIME_TYPE_UNIQUE_ID mime type allows applications to assign
a unique identifer to source surfaces. When surfaces with the same
identifer are used multiple times. The backend can, if supported, only
emit one copy of the image data for mupltiple uses.