Merge of r4979, r4982: image filter configuration inheritance.

Merge of r4979, r4982: image filter configuration inheritance.

*) Image filter: configuration inheritance fixes.

   The image_filter_jpeg_quality, image_filter_sharpen and
   "image_filter rotate" were inherited incorrectly if a directive
   with variables was defined, and then redefined to a literal value,
   i.e. in configurations like

       image_filter_jpeg_quality $arg_q;

       location / {
           image_filter_jpeg_quality 50;
       }

   Patch by Ian Babrou, with minor changes.

*) 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.