feat: Use the available file-size for avatar-images better

The resolution-limits for avatar-images are currently 512x512 or 128x128.
Reducing the resolution further, to 2/3 each step, can reduce the quality
much more than is necessary to fit within the file-size-limits,
which are currently 60 kB or 20 kB.

An image made entirely of noise (which results in unusually
large file-sizes), encoded with jpeg-quality 75,
and 4:2:2-colour-subsampling (the format currently used
for encoding images), can be below 60 kB at 227x227.
For the lower file-size-limit of 20 kB,
such images with a resolution of 128x128 already fit.
More normal images will have a lower file-size at the same resolution.

Before this change, the target-resolutions for resizing were:
512x512 ->                       341x341 ->                       227x227.

And for the lower file-size-limit:
128x128 (does already fit).

After this change, the target-resolutions for resizing will be:
512x512 -> 448x448 -> 392x392 -> 343x343 -> 300x300 -> 263x263 -> 230x230.

And for the lower file-size-limit, those will be:
256x256 -> 224x224 -> 196x196 -> 172x172 -> 150x150 -> 131x131.

The resolution-limit has been increased to 256x256, because the file-size
of many images is still smaller than 20 kB when encoded at 256x256,
and it can be a large improvement in quality.

Author: 72374

src/blob.rs

@@ -483,7 +483,8 @@ impl<'a> BlobObject<'a> {
                             ));
                         }
 
-                        target_wh = target_wh * 2 / 3;
+                        // Note: This is only done for avatar-images.
+                        target_wh = target_wh * 7 / 8;
                     } else {
                         info!(
                             context,

src/constants.rs

@@ -199,7 +199,7 @@ pub const WORSE_IMAGE_BYTES: usize = 130_000;
 // max. width/height and bytes of an avatar
 pub(crate) const BALANCED_AVATAR_SIZE: u32 = 512;
 pub(crate) const BALANCED_AVATAR_BYTES: usize = 60_000;
-pub(crate) const WORSE_AVATAR_SIZE: u32 = 128;
+pub(crate) const WORSE_AVATAR_SIZE: u32 = 256;
 pub(crate) const WORSE_AVATAR_BYTES: usize = 20_000; // this also fits to Outlook servers don't allowing headers larger than 32k.
 
 // max. width/height of images scaled down because of being too huge