Draw an overline as thick as the font's underline (#741)

[gravitypdf-bot]

Fixes #741

Found while working on #552

Problem

text-decoration: overline is drawn at a fixed 0.06 em, whatever the font, so it comes out heavier than the underline on the same text.

$mpdf = new \Mpdf\Mpdf();
$mpdf->WriteHTML('<p style="font-size: 40pt; text-decoration: underline overline">Decorated</p>');

	Browser	Before	After
Underline, 40pt DejaVu Serif Condensed	font's underlineThickness (44/1000)	1.76 pt	1.76 pt
Overline, same text	font's underlineThickness	2.40 pt	1.76 pt
Overline in a table shrunk to fit (autosize)	thinned with the text	2.40 pt, unshrunk	same as the underline beside it

Cause

The // OVERLINE block in Cell() read:

$decorationfontkey = (int) (((float) $this->textparam['o-decoration']['fontkey']) / $shrin_k);
$decorationfontsize = $this->textparam['o-decoration']['fontsize'];

The font key is a string such as dejavuserifcondensed; casting it to a number gives 0, so $this->fonts[0] was looked up, the font's ut and CapHeight were never found, and the 60/1000 and 700/1000 fallbacks were always used. The / $shrin_k that belongs on the size was on the key, so in a shrunk table the overline was measured at the unshrunk size.

Fix

Read the key and size as the underline and line-through blocks do: the key as is, the size divided by $shrin_k. The overline now takes the font's underline thickness, sits at the font's cap height (729/1000 for DejaVu Serif Condensed, rather than the 700 fallback, so it moves up slightly), and shrinks with its table.

Based on gravitypdf: #722 edits the same overline block, but only the lines after these two, so the two apply independently.

Legacy mode

This is a plain bug fix, so it applies in both cssModes.

Tests

• tests/Mpdf/OverlineThicknessTest.php: in both modes, an overline at 40pt is drawn at the font's underline thickness, the same width as the underline, for a TrueType font (DejaVu Serif Condensed, 1.76 pt) and a core font (Times, 2.0 pt); and in a table shrunk to fit, the overline is thinned to the same width as the underline. All fail before the fix.
• Snapshot overline-thickness: DejaVu Serif Condensed and DejaVu Sans Condensed text at 40pt with underline, overline and both, and a table shrunk to fit with both lines; each caption says the lines match. Fails before the fix (page RMSE 0.0587).
• Refreshed fixtures decorations-amendment-legacy, decorations-amendment-standard and decorations-propagate: each draws overlined text, and the only change in their objects is the overline's width (0.720 → 0.528, 0.600 → 0.440) and its height on the page. Checked page by page.

Checks

• composer test: OK, 12739 tests (31 skipped, none of them snapshots; --group snapshot 219 OK)
• composer cs: clean
• PHPStan: same 23 errors as gravitypdf (stale baseline, Regenerate the PHPStan baseline for PHPStan 2.3, so the Lint job passes again #716), none new

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