allow disabling anti-aliasing via cli flag

simply running nsxiv with `--anti-alias` will enable anti-aliasing, and
running it with `--anti-alias=no` will disable it.

the cli flag will overwrite the config.h default.

Closes: https://codeberg.org/nsxiv/nsxiv/issues/349

image.c

@@ -80,7 +80,7 @@ void img_init(img_t *img, win_t *win)
 	img->zoom = MIN(img->zoom, ZOOM_MAX);
 	img->checkpan = false;
 	img->dirty = false;
-	img->aa = ANTI_ALIAS;
+	img->aa = options->aa;
 	img->alpha = ALPHA_LAYER;
 	img->multi.cap = img->multi.cnt = 0;
 	img->multi.animate = options->animate;