lavu/mem: add av_dynarray2_add()

Based on a patch by Clément Bœsch.

See thread:
From: Clément Bœsch <ubitux@gmail.com>
Subject: [FFmpeg-devel] [PATCH 1/5] lavu: add av_dynarray_alloc_elem().
Date: Sun, 14 Apr 2013 03:07:54 +0200