Make it possible to run adjustments and custom processing on all rendered images

[Sebobo]

Description

Currently the ThumbnailService returns the original image if the size is not changed or a larger image is requested than the original.

This makes it impossible to optimize images via the Moc/ImageOptimizer package (mocdk/MOC.ImageOptimizer#14) and similar techniques, as the method refreshThumbnail is never called and cannot be hooked into to do post-processing.

The solution from #1178 is not enough to solve this as it also doesn't allow using custom libraries and behavior.

Expected behavior

The ThumbnailService should have a option to always force the generation of thumbnails and therefore allow post-processing of all images.

Actual behavior

1. Create thumbnail from image which is same or larger than the one requested
2. The original unmodified image is returned and no thumbnail variant generated

Affected Versions

Neos:

2.3
3.0

• Run all images through ThumbnailGenerator class
• Extract processResource method to allow changing the behavior via custom generators
• Allow modification of the $adjustments array in the generator via configuration