A block with max-width and margin: auto renders full width, because min-width and max-width are not read on blocks #588

Part of #552 (step 28, values read wrongly).

max-width and min-width are never read on a block. The common centred-container pattern, max-width with margin: auto, renders the full width of the page.

Measured on gravitypdf at 60b82630, PHP 8.5.4, from the filled rectangles.

Cause. setCSS() reads WIDTH into blk[…]['css_set_width'] (src/Mpdf.php:19356-19359). Nothing reads MAX-WIDTH or MIN-WIDTH for a block; Img reads them for images only (src/Tag/Img.php:138-141). BlockTag::open() works out auto margins only when css_set_width is set (src/Tag/BlockTag.php:830-875). With no width, the block fills its container, and margin: auto computes to 0.

Fix.

• In BlockTag::open(), before the auto margins are worked out, take the width the block would have: css_set_width, or else the container's inner width less margins, borders and padding.
• Clamp that width by max-width, then by min-width. Percentages resolve against the container's inner width, as width does.
• If the clamped width differs, set css_set_width to it, so the existing auto-margin code centres or aligns the block.
• Floats and positioned blocks take the same clamp.

box-sizing: border-box changes what these values measure; that, and min-/max- on tables and cells, is #593. #553 moves centred blocks with a set width on pages of a different width, and this change goes through the same path.

Tests. Each row above, margin-left: auto alone, an rtl block, a float, a positioned block, nested containers, and a block with padding and borders.