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WebP: Fix lossless encoding and reimplement downsampling

Rasyuqa A H requested to merge rasyuqaa/krita:rasyuqaa/webp-fix-lossless into master

Found some more WebP export bugs: lossless mode encoding doesn't seem to produce lossless image, and high bitdepth documents got glitched when exported directly to WebP (on master). This MR attempts to fix those bugs.

  • Lossless encoding fix
Before After
lossless-before lossless-after
  • High bitdepth fix

WebP input is limited to 8bpc, so I tried to reimplement the downsampling method that exists on Krita 5.1.x before. This also reenables the "Dithering" option on export.

Before After
testpic-16b-before testpic-16b-after

Test Plan

  • Build Krita
  • Try to export WebP lossless
  • Also try with high bitdepth (>8bpc) images
  • Verify that it also works with animated WebP

Formalities Checklist

  • I confirmed this builds.
  • I confirmed Krita ran and the relevant functions work.
  • I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
  • I made sure my commits build individually and have good descriptions as per KDE guidelines.
  • I made sure my code conforms to the standards set in the HACKING file.
  • I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list,

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