Background viewer: viewport horizontal position is fixed

[Vulcalien]

I noticed that the red rectangle drawn to indicate the viewport of the background viewer seems to always have the same x coordinate, despite the background scrolling horizontally.

This is also shown in this example, in which scroll is 0, 0 but the rectangle is slightly moved to the right.

[Mireille]

Good catch! I introduced a typo while cleaning up the code last night. Thanks.

[Vulcalien]

I thought I was to late, but nice!