(Ozone) Fix blurry checkmark icons

Created by: jdgleaver

This is a follow-up to #353

As pointed out by @im-4-potato, Ozone's checkmark icons are blurry at large scales:

They are also 'clipped' at the cardinal points, because the non-transparent region extends right to the edges of the PNG images (this is incorrect - there should always be a transparent border).

This PR fixes the issue: