Unable to build with imagemagick 7.x

[adlerweb]

Imagemagick 7.x is stable for a while now and has replaced version 6, which is required to build obs-gphoto, on several distros. Since version 7 has some API changes and legacy 6.x-packages are being removed more and more it is quite hard to build this software on most modern systems. According to the official guide porting should be quite straightforward, however I (and others) got incorrect image sizes when just switching headers, so if imagemagick should be used more work seems to be required.
As an alternative libjpeg, which is preinstalled on most systems, can be used. Dunkelstern already ported the code. I haven't really tested it yet, but the code compiles and obs manages to display a proper image.