This patch makes sure to clear the border buffer of fullscreen view so
the border doesn't get drawn behind a fullscreen view, which would be
visible if the view was transparent.
The borders are implemented as a surface/buffer attached to each view
which is sent to and rendered by wlc in the view_pre_render callback.
All the drawing logic is handled in sway/border.c and all the logic for
calculating the geometry of the border/view is handled in
`update_geometry` in sway/layout.c (same place as gaps are calculated).
