Background now uses actual output geometry

[Timidger]

Background will now ignore the top bar and correctly stretch to the entire length of the screen.

[coveralls]

Coverage increased (+0.3%) to 57.065% when pulling 7fb9c51 on background-bar-size-fix into 799c791 on master.

[SnirkImmington]

Will the top bar cut off the top of the background?
Edit: this is what the PR does 🤦‍
Edit edit: actually can you confirm this first? I think the desired behavior is usually that the background shouldn't be cut off on top. This is the behavior on awesome, for example (I think - all of the pictures I have ATM are too big and cut off anyway...).
I understand if some users don't want the background stretched (i.e. if they have a specific-resolution file) but for me it seems like getting cut off would be more difficult.

If you're okay with the extra work, wanna make this configurable?

[Timidger]

Making this configurable is the way to go, I'll look into it later. Shouldn't be hard to add to this PR.

[Timidger]

Hmm, as I think about this, I still think this should be an option but an option that the background can choose.

The current background program is very very basic, but eventually it should be possible for it to dictate where it should be drawn (and be able to query to see the geometry of the bar, and it can decide what to do with that information).

For now, I'll close this PR so I can be reminded to make this change with how the background is drawn.