Draw fullscreen_bg on the correct layer

[fbushstone]

Closes Issue 487.
dwl would draw the background on the wrong layer, obscuring SDL apps such as Quake and making certain terminals totally transparent except for the text (adjusting alpha on fullscreen_bg would not have helped in this case).

[sevz17]

Hm
I don't think this will work, this could render the background behind the wallpaper (I didn't test).
And I think it's better to avoid fullscreen windows being in other layer than LyrFS and make sure that fullscreen_bg is always at the bottom of LyrFS

[fbushstone]

On second inspection, I shouldn't have pushed that change. It seems like certain clients request fullscreen mode, are actually fullscreen, but then dwl doesn't consider them fullscreen internally for whatever reason. Toggling fullscreen twice seems to get around this for now.

[fbushstone]

Poor solution on my part, see my updated pull request