add get-list-of-field-names

[jcupitt]

You use #get_value to get a metadata field, but there's no way to get a list of all field names.

irb(main):008:0> x = Vips::Image.new_from_file "img_0254.jpg"
=> #<Vips::Image:0x2843760 ptr=0x26f31b0>
irb(main):002:0> x.get_value "exif-data"
=> "Exif\x00\x00II*\x00\b\x00\x00\x00\t\x00\x0F\ .....

You can get gobject properties like this:

irb(main):017:0> x.class.properties
=> ["nickname", "description", "width", "height", "bands", "format", "coding", "interpretation", "xres", "yres", "xoffset", "yoffset", "filename", "mode", "kill", "demand", "sizeof-header", "foreign-buffer"]

But that won't list vips metadata fields.

See eltiare/carrierwave-vips#25

[jcupitt]

We now have vips_image_get_fields() ... just call this.

jcupitt/libvips@85be55f

In 8.5.

[jcupitt]

Works in Python:

>>> x = Vips.Image.new_from_file("wtc.jpg")
>>> x.get_fields()

But not in Ruby? Strange, I guess gchar** is not a supported return type.

[jcupitt]

Added in 2.0.5