Draw a position: fixed block on every page, and stack positioned blocks by z-index #601

Part of #552 (step 28, missing support, medium priority).

Two faults with positioned blocks:

• A position: fixed block is drawn on the page where it appears in the source only. CSS 2.1 §9.3.1 says that in paged media a fixed box "is rendered on every page". That makes position: fixed the portable way to put a letterhead, a watermark or a running footer on every page.
• z-index does not decide which positioned block is on top. Only the text is reordered; the backgrounds stay in document order.

Nested positioning is #599.

Measured on gravitypdf at 60b82630, PHP 8.5.4.

Cause.

• Fixed. A positioned block's HTML is saved with the page it appeared on (src/Mpdf.php:14883). It is drawn once, on that page, when WriteHTML() finishes (:15139-15146).
• z-index. It becomes an optional-content layer around the block's content (src/Tag/BlockTag.php:506, BeginLayer() at src/Mpdf.php:3389). _enddoc() moves each layer to the end of its page, ordered by z-index (src/Mpdf.php:11102-11130). A block's background is not written inside its layer: PaintDivBB() writes it at the page's background marker, so only the text moves up. In the probe, the page stream is the green fill, the red fill, y, then /OC /ZI5 BDC with x alone.

Fix.

• Fixed. Keep fixed blocks apart from absolute ones, and draw each one on every page from the page it first appears on, as a header is. A document that uses a top-level fixed block to mean "here, on this page" today changes output. Say so in the PR. Decide whether a configuration option keeps today's behaviour, or whether position: absolute is the answer for that case.
• z-index.
  • Paint each positioned block's background, borders and shadow inside its own layer, with its content.
  • Order positioned blocks by z-index, then by document order, with auto and 0 together.
  • Negative values go below the in-flow content, which needs a layer below it.
  • _enddoc() already sorts the layers. What is missing is putting everything a positioned block draws inside its own layer.
  • In-flow blocks with z-index are reordered today, which CSS does not do for boxes that are not positioned. Decide whether to keep that. Existing documents may rely on it.

Tests. A fixed block on a one-page and a six-page document, a fixed block after a forced page break, a fixed block with % offsets on pages of different sizes, and z-index of 5, 0, auto and −1 between two overlapping positioned blocks. Read the order of the fills from the page stream.