Introduce switch for the drawing backend

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