Medium image height can be wrong with GD

[kamil4]

Detailed description of the problem

For pictures with a ratio smaller than 16:9, when using GD, the height of the generated medium picture can exceed the default 1080.

Steps to reproduce the issue

Make sure that imagick is not being used.
Add a new picture with the resolution of 4608x3456 (4:3 aspect ratio).
Check the resolution of the generated medium picture: it will be 1920x1440 instead of the expected 1440x1080.

Output of the diagnostics (Settings => Diagnostics)

Lychee Version (json): 3.2.12
DB Version: update_030212
System: Linux
PHP Version: 7.2
MySQL Version: 50724
Imagick: -
Imagick Active: 0
Imagick Version: -
GD Version: bundled (2.1.0 compatible)
Plugins:

Browser and system

Firefox 64.0.2 on Linux