Image filter: fixed image_filter rotate inheritance.

Image filter: fixed image_filter rotate inheritance.

Configurations like

    location /i/ {
        image_filter resize 200 200;
        image_filter rotate 180;

        location /i/foo/ {
            image_filter resize 200 200;
        }
   }

resulted in rotation incorrectly applied in the location /i/foo, without
any way to clear it.  Fix is to handle conf->angle/conf->acv consistently
with other filter variables and do not try to inherit them if there are
transformations defined for current location.