Add override for GdkPixbuf.Pixbuf.save
Pixbuf.save wasn't usable due to being a variadic function. In addition,
both its C interface and Pixbuf.savev are unpythonic, taking keys and
values separately. This adds a Pixbuf.savev override taking an optional
dict for options. It can be used like pixbuf.save('out.png', 'png')
or
pixbuf.save('out.png', 'png', {'tEXt::key': 'value'})
.