**Reviewers:** Likely quicker to review commit-by-commit.
**Summary**
In preparation for adding a PPTX `Picture` shape _sub-partitioner_,
extract management of PPTX partitioning-run options to a separate
`_PptxPartitioningOptions` object similar to those used in chunking and
XLSX partitioning. This provides several benefits:
- Extract code dealing with applying defaults and computing derived
values from the main partitioning code, leaving it less cluttered and
focused on the partitioning algorithm itself.
- Allow the options set to be passed to helper objects, prominently
including sub-partitioners, without requiring a long list of parameters
or requiring the caller to couple itself to the particular option values
the helper object requires.
- Allow options behaviors to be thoroughly and efficiently tested in
isolation.
