Add option to crop a burst precisely via `firstValidSample` and `lastValidSample`

[alexamici]

Useful enhancements:

• read the firstValidSample and lastValidSample tags and expose them (possibly making them more readable)
• provide and option to crop the burst to the border of the actual image

[lanougue]

Hello @alexamici ,
I would be strongly interested by this possibility. Do you plan to develop this in the coming time ?
Thanks

[alexamici]

Not in the near future unfortunately: -ENOTIME.

We may need the feature by end 2022, but no promise.