avcodec/pngdec: do not blend on transparent black

There is no need to memset the zlib output buffer, as there is no
blending happening there. Instead, do not blend when the dispose
operation is set to 'background' (tranparent black).

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pngdec: allow for some code path optimizations.

Use 'if (CONFIG_APNG_DECODER)' where needed, so that the compiler can
optimize out some portion of code.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/ppc/h264qpel_template: protect unistd.h by #if HAVE_UNISTD_H

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>