Introduce switch for the drawing backend

This commit restores the old XCB drawing code paths while keeping the
cairo drawing available via a compile-time switch (I3BAR_CAIRO). This
is necessary as cairo currently has a bug that breaks i3bar for users
without the RENDER extension, which might be the case, e.g., for VNC
users.

For more context, see #1989 and the discussions about its fix. Once the
cairo fix is available in a stable release, i3 can depend on that version
and remove the XCB drawing code paths.

fixes #1989

i3bar/include/outputs.h

@@ -13,7 +13,7 @@
 #include <cairo/cairo-xcb.h>
 
 #include "common.h"
-#include "cairo_util.h"
+#include "draw_util.h"
 
 typedef struct i3_output i3_output;