render_root: fix popup_during_fullscreen logic
render_root: fix popup_during_fullscreen logic

The first issue is that there seems to be a typo: fullscreen->window
should have been child->window. The corrected check is redundant since
the while loop checks if the transient_con has a window.

The second issue is that popup_during_fullscreen is never checked even
though the behaviour should be exclusive to the "smart" option.

Airblader/i3,stapelberg/i3,Airblader/i3,stapelberg/i3,i3/i3,stapelberg/i3,stapelberg/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3-original,i3/i3,i3/i3,i3/i3,Airblader/i3