I'm afraid that as far as I'm aware, it's not possible to read back from the LCD's memory. It's write-only. So there's actually no way to get a screenshot of what's currently on it. The best you could do is maybe create a new g which was an offscreen buffer, and write to that - but obviously it'd have to be a lower bit depth.
I'd love to be proven wrong about the LCD reads though. Some tech data is here: https://www.espruino.com/Bangle.js+Technical
Basically the ST7789 is set up in the 8 bit parallel mode, which would be ok for reads except I'm pretty sure that the read/write lines are hard-coded to always be set to write (although I guess it's possible they are on the IO expander).
8 bit parallel
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