lavfi: use min_perms and rej_perms for out pads.

There are several reasons for doing that:

1. It documents the code for the reader and helps find
   inconsistencies and bugs.

2. For rej_perms, it guarantees the change will be done
   even if the output reference can be created by several
   code paths.

3. It can be used to predict cases where a copy will,
   or will not happen and optimize buffer allocation
   (for example not request a rare direct-rendering buffer
   from a device sink if it will be copied anyway).

Note that a filter is still allowed to manage the permissions
on its own without using these fields.

libavfilter/avfilter.h

@@ -240,22 +240,29 @@ struct AVFilterPad {
     enum AVMediaType type;
 
     /**
+     * Input pads:
      * Minimum required permissions on incoming buffers. Any buffer with
      * insufficient permissions will be automatically copied by the filter
      * system to a new buffer which provides the needed access permissions.
      *
-     * Input pads only.
+     * Output pads:
+     * Guaranteed permissions on outgoing buffers. Any buffer pushed on the
+     * link must have at least these permissions; this fact is checked by
+     * asserts. It can be used to optimize buffer allocation.
      */
     int min_perms;
 
     /**
+     * Input pads:
      * Permissions which are not accepted on incoming buffers. Any buffer
      * which has any of these permissions set will be automatically copied
      * by the filter system to a new buffer which does not have those
      * permissions. This can be used to easily disallow buffers with
      * AV_PERM_REUSE.
      *
-     * Input pads only.
+     * Output pads:
+     * Permissions which are automatically removed on outgoing buffers. It
+     * can be used to optimize buffer allocation.
      */
     int rej_perms;