Moved SSE2 optimization code into the main libpng source directory.
Configure libpng with "configure --enable-intel-sse" or compile
libpng with "-DPNG_INTEL_SSE" in CPPFLAGS to enable it.
Added code for PowerPC VSX optimisation (Vadim Barkov).
Avoid potential overflow of shift operations in png_do_expand() (Aaron Boxer).
OBS-URL: https://build.opensuse.org/package/show/graphics/libpng16?expand=0&rev=92
