feat: Optimize avatar size multiplier for 2 << n px avatars

Instead of 2/3 which is not optimal for 512 px avatars usually passed to Core, use the sequence 3/4,
5/8, 4/8, 3/8... to do "smaller" downscaling steps and reduce aliasing effects.

Before, it was discussed that just 3/4 can be used. However, if we repeat the reduction step, we get
`3 << n` as a numerator and this way increase aliasing effects on each step. Better limit the
numerator to 5.

src/blob.rs

@@ -435,6 +435,8 @@ impl<'a> BlobObject<'a> {
                         });
 
             if do_scale {
+                let (mut m, mut d) = (3, 4);
+                let wh = target_wh;
                 loop {
                     if mem::take(&mut add_white_bg) {
                         self::add_white_bg(&mut img);
@@ -467,8 +469,11 @@ impl<'a> BlobObject<'a> {
                                 "Failed to scale image to below {max_bytes}B.",
                             ));
                         }
-
-                        target_wh = target_wh * 2 / 3;
+                        target_wh = wh * m / d;
+                        (m, d) = match m > 3 {
+                            true => (m - 1, d),
+                            false => (5, d * 2),
+                        };
                     } else {
                         info!(
                             context,

src/blob/blob_tests.rs

@@ -4,7 +4,9 @@ use super::*;
 use crate::message::{Message, Viewtype};
 use crate::param::Param;
 use crate::sql;
-use crate::test_utils::{self, AVATAR_64x64_BYTES, AVATAR_64x64_DEDUPLICATED, TestContext};
+use crate::test_utils::{
+    self, AVATAR_64x64_BYTES, AVATAR_64x64_DEDUPLICATED, TestContext, TestContextManager,
+};
 use crate::tools::SystemTime;
 
 fn check_image_size(path: impl AsRef<Path>, width: u32, height: u32) -> image::DynamicImage {
@@ -239,6 +241,22 @@ async fn test_selfavatar_in_blobdir() {
     );
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_huge_selfavatar() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let t = &tcm.unconfigured().await;
+    let avatar_src = t.get_blobdir().join("avatar.png");
+    let bytes = include_bytes!("../../test-data/image/noise400x400.png");
+
+    fs::write(&avatar_src, bytes).await?;
+    t.set_config(Config::Selfavatar, Some(avatar_src.to_str().unwrap()))
+        .await?;
+    let avatar_cfg = t.get_config(Config::Selfavatar).await?.unwrap();
+    // At 3/4 the avatar is still huge, so it's downscaled to 5/8.
+    check_image_size(avatar_cfg, 250, 250);
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_selfavatar_copy_without_recode() {
     let t = TestContext::new().await;