[bug] background image rendering not taking screen rotation into account

## I'm submitting a…

[x] Bug

[ ] Feature Request

[ ] Other (Please describe in detail)

## Current Behavior
Whenever I use monitor setup with screen orientation other than 'normal' (e.g. rotated right), the background image is rendered with a wrong width

## Expected Behavior
the bg image rendering counts the screen orientation in and e.g. in setup with screen rotated 90 degrees, it works with screen resolution's height instead of width

## Reproduction Instructions
just use some screen manager (i use `arandr`) to rotate your screen 90 degrees.
Reproducible on single monitor setup as well

## Environment
Output of `i3lock --version`:

i3lock: version 2.14.1 © 2010 Michael Stapelberg