PNG window bits defaults to 9 instead of defaulting to 15 like in libpng.

https://github.com/boostorg/gil/blob/d8c56358fae6302dd2f2e8cc496bcd4535c16183/include/boost/gil/extension/io/png/tags.hpp#L218

The default window bits here should be 15 for a 32k window, to match libpng. Having it set to 9 makes compression much much slower for anyone who uses png_tag with write_view, and there is no stated reason for why this was set to 9.