Improve image caching

[Astrashh]

Pull Request

Brief description.

Adds parameters to pack.yml that provide more control over image caching behaviour:

• images.cache.load-on-use - If set to true, images will load into memory upon use rather than on pack load.

• images.cache.timeout - How many seconds to keep images loaded in the image cache for. If set to a number greater than 0, images will be removed from memory if not used after the timeout, otherwise images will stay loaded in memory. Setting the timeout to greater than 0 will trade decreased memory consumption when not performing any image reads for a period of time for extra processing time required to perform cache lookups.

What Issues Does This Fix?

421

Licensing

• [x] I am the original author of this code, and I am willing to release it under GPLv3.
• [ ] I am not the original author of this code, but it is in public domain or released under GPLv3 or a compatible license.

Types of changes

• [ ] Bug Fix
• [ ] Build system
• [ ] Documentation
• [ ] New Feature
• [x] Performance
• [ ] Refactoring
• [ ] Repository
• [ ] Revert
• [ ] Style
• [ ] Tests
• [ ] Translation

Compatiblity

• [ ] Breaking change
• [x] Non-Breaking change.

Contribution Guidelines.

• [x] I have read the CONTRIBUTING.md document in the root of the git repository.
• [ ] My code follows the code style for this project.

Documentation

• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.

Testing

• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[duplexsystem]

You could simply the images.cache.unload-on-timeout and images.cache.timeout keys into just one key where zero is no timeout and any other number is a time out of that long. Should make configuring this easier.