Add support for ImageMagick version 7

[vouillon]

libvips uses specific loaders for ImageMagick 7.

[coveralls]

Coverage remained the same at 100.0% when pulling 0f5054f on besport:imagemagick-7 into 0f58f95 on lovell:master.

[lovell]

La magie, merci/thanks!

[jcupitt]

This is not very important, but checking for the exact class names feels a little ugly, and could break.

How about checking the nickname instead? The libvips base object has a nickname class member:

https://github.com/libvips/libvips/blob/master/libvips/include/vips/object.h#L525

It's also exposed as an object member and glib property on vips objects. The nickname is the thing that's used to look up operations by name, eg.:

GType vips_type_find( const char *basename, const char *nickname );
const char *vips_nickname_find( GType type );

vips_type_find uses a lazy hash to go from a nickname to a GTypeefficiently.

In this case you could use:

char const *load = file ? vips_foreign_find_load(file) : NULL;
GType gtype = load ? g_type_from_name(load) : 0;
const char *nickname = gtype ? vips_nickname_find(gtype) : NULL;

And nickname will be something like magickload or NULL. libvips guarantees that nicknames are unique and unvarying, ie. they are a supported part of the API (unlike class names, which are not). You could perhaps use a hash to map nicknames to file types and save the switch.

[jcupitt]

... I could try a PR, if that would help.

[lovell]

@jcupitt Great idea, thank you, I've added this via commit 8da30f0

[jcupitt]

Ah, much prettier! Nice!