Convert the `renderTasks`, used in `PDFPageProxy.render`/`PDFPageProxy.getOperatorList`, to a Set
Convert the `renderTasks`, used in `PDFPageProxy.render`/`PDFPageProxy.getOperatorList`, to a Set

When removing tasks we're currently forced to *indirectly* iterate through the array, which can be avoided by using a Set instead.
Furthermore, we can also (slightly) modernize the code responsible for initializing the `renderTasks`.

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