Add isProgressive flag to metadata output, requires libvips v8.7.0 #1186
Description
http://sharp.pixelplumbing.com/en/stable/api-input/#metadata
isProgressive: Boolean indicating whether the input image is interlaced (supported by JPEG and PNG)
See commit jcupitt/libvips@15abbbe
("progressive" is used in preference to "interlaced" to ensure consistency with the output functions.)