[Feature Request] Retain X previous darkroom images in RAM for quick comparisons

If RAM is available and perhaps by user configuration, it would be nice to retain X images in RAM to be able to quickly scroll through them in the darkroom.

Most users now have at least 16GB of RAM, some even more. Serious photographers or hobbyists have strong machines, and it is a waste to leave RAM free. Utilizing RAM ahead of time, when large data is concerned, will almost always beat most algorithms or optimizations.

This would massively improve workflows when comparing images for sharpness or other qualities. Waiting one second for the newly selected image to render is too slow for any quick work, and a waste of energy to constantly re-render the image.

Additionally, it would also be nice to be able to automatically pre-render the next image in the darkroom as well, so when switching it for the first time, it would load instantly.

So in summary:

• 1. Be able to configure darktable to retain X previous darkroom images in RAM;
• 2. Be able to configure darktable to pre-render selected+Y images in RAM.

No files or tests are named in the issue. Start by locating the darkroom image-selection and rendering paths, then inspect existing cache and configuration handling. Done means configurable retention of previous images and pre-rendering of selected plus Y images, with quick comparisons without repeated rendering.