[RFC] Don't export imshow/imshow256/imshow24bit

[johnnychen94]

Compared with the standard show method, I find myself rarely use these functions, perhaps it's a good idea to not export them.

I don't have plans on doing it recently, just open this issue to see how others feel about it. (For example, with 👍 or 👎)

[t-bltg]

Closed by #58.