Fixes some very visible bugs.

(v6.1 is retracted)

Fixes since v6

• Fix an assertion failure in map_win() (happens when using debug build of compton with i3-gaps) #143
• Fix a problem where the content of a closed window is left on the screen. #140 #82 (comment)
• Fix a problem where the content of a closed window is flashing on the screen when --use-damage is used. #82 (comment)