Clear framebuffer before drawing borders (#4308)
Clear framebuffer before drawing borders (#4308)

Clear framebuffer before copying surface to prevent visual garbage

Fixes #3481
Corrects problems of #4280
Airblader/i3,i3/i3,Airblader/i3,Airblader/i3,i3/i3,i3/i3,Airblader/i3-original,stapelberg/i3,stapelberg/i3,stapelberg/i3,i3/i3,stapelberg/i3,Airblader/i3-original,Airblader/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3-original