Target DPI resolution for images

[Omikhleia]

A bit messy and poorly documented:

• The resilient classes have a resolution class option, undocumented (well, actually just mentioned in passing -- i.e. in a footnote -- in the documentation of the embedders package, which is the only one to use it at this point...)
• The printoptions.sile collection has a resolution setting...
• The embedders.sile collection uses one or the other depending on what it finds...
• The standard image package obviously doesn't know about it and doesn't warn (at least) for under-resolved images (over-resolved images are what printoptions aims at addressing, hence the "at least")

There's probably ways to better document and address the need!