Currently for any screen with a ratio 1.6 or above, the preview uses a 16:10
image. However, the majority of Fx users have a screen that is 16:9[0], so for
most users the preview shows images distorted (compressed horizontally).
Originally I just added a new 16:9 version of the monitor image, but then I
realised I could save on filesize _and_ make it responsive to whatever the
user's screen actually is, rather than using arbitrary presets, by using
border-image.
The new image files are just sliced up versions of the original monitor.png
files, zopfli compressed to match (though with the power indicator dropped from
the Linux/Windows version to avoid distorting it). The combined filesize
savings seem to be 8.5KiB on macOS and 6.5KiB on Linux/Windows.
With the removal of the use of margins on the canvas we no longer need the
platform-specific setDesktopBackground.css file.
[0] https://data.firefox.com/dashboard/hardware
As of 3rd March 2019 the top three resolutions, 1366x768, 1080p, & 1600x900,
are all 16:9 and make up 67% of the userbase.
Differential Revision: https://phabricator.services.mozilla.com/D23114
4843dbc900