Before, extensions were missing the definition of mandatory methods.
At the moment, it's not a problem because the coding guidelines are not
enforced by the code. But in the future, it will break.
Now, extensions have all mandatory method definitions.
This PR adds additional parameters to the ImageProxy extension such as to allow easy proxying via Apache mod_rewrite (which: 1. needs a hard-coded absolute path ; 2. can not URL-decode passed URL)
See the updated README.md for further information (and corresponding Apache configuration example).
I've taken care to remain backward-compatible with existing setups; in particular, the (former) force parameter will automatically be migrated to the (new) scheme_https counterpart.
