Add a utility function to report if payload will fit into image before encoding

[nervetrip]

Would it be possible to add a utility function that would report if a given payload will fit inside a given image? Perhaps it could even report how many copies of it would fit?

I've found that larger payloads will sometimes error out when encoding into an image. I'm currently catching that error, reducing the number of copies, and then encoding again. It seems like doing some simple checks up front would be more optimal.

Perhaps there may be other information that might be useful before attempting to encode, but I'm not sure what that might be as I'm not too familiar with this library yet.

[Jack-Works]

yes, it is possible but needs some math, maybe the initial developer can help cc @guanbinrui