Align inline text with vertical-align keywords, and indent cells and hanging lines with text-indent #608

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

Two inline-layout gaps:

• vertical-align on inline text. middle, top, bottom, text-top and text-bottom do nothing; they work on images.
• text-indent. It is ignored in table cells, and hanging or each-line drop the whole declaration.

Icons set beside text with vertical-align: middle, and hanging indents in references and numbered clauses, come out wrong.

Measured on gravitypdf at 60b82630, PHP 8.5.4, from the Td operators and the positions text is drawn at.

Cause.

• vertical-align. setCSS() handles SUPER, SUB and BASELINE. Anything else goes to the length branch, where SizeConverter reads a keyword as 0 (src/Mpdf.php:19515-19550). Images take the keywords from Tag::ALIGN (src/Tag/Tag.php:74-85), and the line code places them.
• text-indent. setCSS() stores it only for blocks, not cells (if ($type != 'INLINE' && $type != 'TABLECELL'), src/Mpdf.php:19377-19381), and Td does not read it. The line code applies it only when !$is_table (:7547, :7749, :8907, :9505). A keyword makes the value unparseable. After Drop a declaration mPDF cannot read so the value it would replace still applies #565 the declaration is dropped; today it converts to 0.

Fix.

• vertical-align keywords on text. They depend on the line box, which is only known once the line is complete.
  • Mark the run with the keyword, as super and sub mark it in textvar.
  • When the line's height is worked out (_setInlineBlockHeights(), src/Mpdf.php:12364), place the run: middle from the parent's baseline and x-height; text-top and text-bottom from the parent's font box; top and bottom from the line box once the rest is placed.
  • A run with top or bottom can make the line taller, which the same pass accounts for.
  • line-height on an inline element does not change the height of its line #548 changes how inline boxes size the line; do this after it, or in the same code.
• text-indent in cells. Read it in Td::open() onto the cell, inherit it into the cell's blocks, and drop the !$is_table conditions so the cell's first line is indented.
• hanging and each-line. Accept the keywords with a length.
  • hanging indents every line except the first.
  • each-line also indents the first line after each forced break (<br>).
  • Both reuse the line code's first-line check with the condition inverted or widened.
  • Drop a declaration mPDF cannot read so the value it would replace still applies #565's validation must let these keywords through.

Tests. Each row above, vertical-align with a length and a percentage (unchanged), middle on a span containing an image, top in a line with a taller image, text-indent in an rtl cell, hanging with a negative length, and each-line after <br>.