Original commit message from CVS:
Patch by: Alessandro Decina <alessandro at nnva dot org>
* ext/libpng/gstpngenc.c: (user_write_data), (gst_pngenc_chain):
* ext/libpng/gstpngenc.h:
Preallocate the output buffer so that g_memdup() and
gst_buffer_merge() aren't needed anymore. This greatly improves
performances and fixes#512544.