Bugfix: Fix resizing of floating windows in borderless/1-px-border mode (Thanks Grauwolf)

Bugfix: Fix resizing of floating windows in borderless/1-px-border mode (Thanks Grauwolf)

Calculations were wrong (they simply didn’t take into account that
there is more than one border style, the code was from before we
implemented that…). We cannot directly set child_rect to the coordinates
as resize_client takes rect and calculates the child_rect, so we need
the new lines of code for this bugfix in any case (rect needs to be
updated).