feat: 3656 - optimized JPEG compression

[monsieurtanuki]

New file:

• image_compute_helper.dart: Optimized methods for BMP and JPEG file computations.

Impacted files:

• background_task_image.dart: now using new optimized saveJpeg method
• new_crop_page.dart: now using new saveBmp method for performances; added a progress indicator; refactored
• pubspec.lock: wtf
• pubspec.yaml: added package flutter_image_compress; refactored
• rotated_crop_image.dart: fixed a bug regarding dispose

What

• Image upload operation could appear to be slow.
• The main reason is that the standard flutter image JPEG compression method is slow.
• With this PR
  • we use the native JPEG compression methods, that are way faster.
  • for the "transient" file, we even save the file as BMP, which is faster to encode.
  • In addition to that, we now display a progress indicator when we click on the "Send image!" button, for a better UX.
• UX could be improved using isolates (no app freezing), but I currently could not find time for this.

Part of

• Privacy review of server-side cropping #3656

[monsieurtanuki]

Note to myself: use isolates for image compression with the new flutter 3.7 feature: background isolate channels.

[M123-dev]

Looks great, thanks @monsieurtanuki

Also the new isolate changes look promising

[monsieurtanuki]

Thank you very much @M123-dev for your review!