avatar: Use a better linear function for padding around the initials

The new function grows quicker and therfore it makes the initials
smaller on a specifc avatar size. It also makes it simpler to adjust the
grow of the padding.
11 jobs for avatar_small_fixes in 16 minutes and 11 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #258521
librem5
api-visibility

00:02:28

passed #258523
librem5
build-debian-clang

00:03:31

passed #258522
librem5
build-debian-gcc

00:02:46

passed #258524
librem5
build-fedora-gcc

00:04:11

 
  Test+Docs
passed #258525
librem5
build-gtkdoc

00:02:04

passed #258526
librem5
unit-test

00:02:00

43.7%
 
  Package
passed #258527
librem5
package-debian-buster

00:01:59

passed #258528
librem5:arm64
package-debian-buster:arm64

00:06:12

passed #258529
flatpak
package-flatpak

00:01:38

 
  Test Package
passed #258530
librem5
autopkgtest-debian-buster-package

00:03:42

passed #258531
librem5
lintian-debian-buster-package

00:01:21