feat: Use resize when resampling images with up to 2.56 megapixels

[72374]

The thumbnail-function can resample images quickly, but with very noticeable aliasing when it is used to resample images to a close resolution (~80% of the original resolution, for example). Even if the resolution-difference is large, the quality of the resampling is rather flawed. The function is for quickly resampling lots of images into tiny preview-images, for which the quality is not particularly important, and the resolution-difference is usually large (~20% of the original resolution, for example).

The amount of processing (and thus time) required to resample images, depends on the resolution of the original image.

By using the thumbnail-function only for images with a high resolution, many images will be resampled with the better resampling of the resize-function, while the processing-time, when resampling images with a high resolution, will still be relatively short.

[iequidoo]

Just in case, the previous discussion is at #6815

[72374]

One question that might be relevant for this PR:
I saw that the image-editor of Delta Chat for Android has 2 resolution-limits, and those are either higher than one or both media-quality settings in Chatmail Core.
Do those apply to the image given to Chatmail Core, or is it only for display in the editor?

If those apply to the image, that could result in downscaling twice; first by the image-editor, to 1536x1536, and then by Core, to 1280x1280, and it would also mean that the change in this PR would apply to all images edited in the Android-app, because the upper limit is 1536x1536.
If that is the case, it could be a good idea to make the limits identical, so that downscaling is only necessary once; which would improve quality and reduce processing-time. But if those should not be changed, the limit in this PR should probably be reduced to 1400x1400 or less, because otherwise this change would effectively be identical to always using bilinear-resampling for the Android-app of Delta Chat.

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I set this PR back to draft, because #7822 can change what is appropriate to do in this PR. With those changes, images with a resolution of up to 1536×1066¹ would not be resampled, because that is a lower resolution than 1280x1280².

Footnotes

1. 768×533 for the lower limit in the image-editor. ↩

2. 640x640 for the lower media-quality-setting. ↩