Improve extendability of AbstractGalleryProcessor, ScalingGalleryProcessorPlugin

On behalf of a plugin [1] that creates (non-thumbnail) image variants in the background, request to better extendability of

• org.hippoecm.frontend.plugins.gallery.processor.ScalingGalleryProcessorPlugin for the instantiation of the processor and for unnecessary duplication of constants (need to be protected for the background processor module).
• org.hippoecm.frontend.plugins.gallery.processor.AbstractGalleryProcessor#makeImage - now we have more a fork than an override

Also see HIPFORGE-101, CMS-10555.

[1] https://github.com/onehippo-forge/gallery-background-processor