Don't interfere with buffer-face-mode.

I'm using `variable-pitch-mode` for all text-mode buffers. Auto-dim-other-buffers breaks this when using buffer-face-set.

My solution to this is using face-remap-add-relative instead of buffer-face-set, but there could be others and I haven't done extensive testing on this solution except that it seems to work as intended for me.

buffer-face-set overrides any faces used by buffer-face-mode (e,g, using variable-pitch-mode) and disables all those faces. Using face-remap-(add|remove)-relative allows us to add the auto-dim-other-buffers-face alongside faces used by buffer-face-mode.