Profile and reduce memory usage for large-image rendering scenarios

[davidnmbond]

Deferred from phase 8 steps 8.4.1 through 8.4.3.

Scope:

• Profile memory usage on documents with many large images
• Implement image streaming so all images are not retained in memory simultaneously
• Implement layout/page disposal after rendered output is emitted
• Re-check peak memory against project targets

Acceptance criteria:

• A repeatable memory-profile scenario exists
• The renderer no longer retains all large images for the full document lifetime
• Page/layout data can be released after use where appropriate
• Findings and limits are documented