x_draw_decoration: always clear parent pixmap when rendering the first child
x_draw_decoration: always clear parent pixmap when rendering the first child

Previously, i3 used to clean the parent pixmap only if the first child
has a titlebar.

This worked well for case `V[a b]` when both `a` and `b` have titlebars
(clearing is required), or both haven't (clearing is not required).
However, in case if `a` has no titlebar, but `b` has, this could lead
to graphical glitches.

This commit rearranges the order of conditions to make sure that the
parent pixmap is cleared regardless of the border style of the first
child.

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