Implement borders

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).

include/border.h

@@ -0,0 +1,10 @@
+#ifndef _SWAY_BORDER_H
+#define _SWAY_BORDER_H
+#include <wlc/wlc.h>
+#include "container.h"
+
+void render_view_borders(wlc_handle view);
+void update_view_border(swayc_t *view);
+int get_font_text_height(const char *font);
+
+#endif