Decoding freezes UI

[NaikSoftware]

Decoding (math operations) proceeds in UI isolate. This cause lags when scrolling list with images (like Instagram feed).

[ahalunetsvisionable]

@NaikSoftware You can check my MR
In dart 2.15 was released isolates groups. Its better than to use worker_manager dependency.

[Solido]

Thank you guys for taking care of the best way to handle decoding.
Update has been pushed mixing your suggestions.
Can you confirm it is OK and then I'll close.
Regards