[libpng16] Avoid up-cast warnings in pngvalid.c. On ARM the alignment

requirements of png_modifier are greater than that of png_store and as a
consequence compilation of pngvalid.c results in a warning about increased
alignment requirements because of the bare cast to (png_modifier*). The code
is safe, because the pointer is known to point to a stack allocated
png_modifier, but this change avoids the warning.

  Fixed default behavior of ARM_NEON_API. If the ARM NEON API option was
    compiled without the CHECK option it defaulted to on, not off.

  Check user callback behavior in pngunknown.c. Previous versions compiled
    if SAVE_UNKNOWN was not available but did nothing since the callback
    was never implemented.

  Merged pngunknown.c with 1.7 version and back ported 1.7 improvements/fixes

pngrutil.c

@@ -1287,7 +1287,9 @@ png_handle_sRGB(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
       return;
    }
 
+#ifdef PNG_sRGB_SUPPORTED
    (void)png_colorspace_set_sRGB(png_ptr, &png_ptr->colorspace, intent);
+#endif
    png_colorspace_sync(png_ptr, info_ptr);
 }
 #endif /* PNG_READ_sRGB_SUPPORTED */