Allow using "vips" driver

[PabloKowalczyk]

Q	A
Branch?	2.x
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	fix #1494
License	MIT
Doc

[dbu]

for phpstan we should add imagine-vips to the require-dev section in composer. (we could also ignore the error, but i prefer adding it, so we notice if something goes off.)

[PabloKowalczyk]

I've added rokka/imagine-vips to require-dev and to suggest also.

[dbu]

ah dang. we support older php than vips does. i will try something with the ci.

[PabloKowalczyk]

ah dang. we support older php than vips does. i will try something with the ci.

PHP 7.1 is quite old https://www.php.net/supported-versions.php :)

[coveralls]

Coverage increased (+0.1%) to 81.682% when pulling 31f67e8 on PabloKowalczyk:add-vips into ee15813 on liip:2.x.