It turns out that `getTextContent` suffers from *similar* problems with repeated images as `getOperatorList`; please see the previous patch.
While only `/XObject` resources of the `Form`-type will actually be *parsed* in `PartialEvaluator.getTextContent`, since those are the only ones that may contain text, we're still forced to fetch repeated image resources where the name differs (but not the reference).
Obviously it's less bad in this case, since we're not actually parsing `/XObject`s of e.g. the `Image`-type. However, you still want to avoid even fetching the data whenever possible, since `Stream`s are not cached on the `XRef` instance (given their potential size) and the lookup can thus be somewhat expensive in general.
To address these issues, we can simply replace the exiting name-only caching in `PartialEvaluator.getTextContent` with a new cache backed by `LocalImageCache` instead.
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