Fixes T10234. This is a more thorough fix.

Root issue is that some time around D13589, we started hitting an object cache for loadCustomInlineRules(), but didn't adjust the code to account for that.

So if a page created multiple similar engines, we'd return the same $rule object for multiple engines, call setEngine() on it with different engines, and then possibly try to render using an already-expired engine the second time through.

Instead, create a separate $rule object for each separate $engine.

Repro is something like this:

• Create a custominlinerule which uses an engine.
• Purge the remarkup cache.
• Load a page which uses the rule in two engines (e.g., in a revision description, and also in an inline comment).
• Before change: second one could fatal. After change: clean load.