Fix icorrect aspect ratio on images with thick borders
Images get squashed vertically when a relatively thick border is enabled on them.
To reproduce insert a long (e.g. slide width) and narrow (e.g. half-a-square) horizontal image with a square grid on it, duplicate, apply a 4px border with 8px corner radius to one copy, and compare the two images visually.
The image with border will appear squashed vertically. The expected outcome is for the image to retain aspect ratio, as requested by the corresponding checkbox in image properties.