Check a font is flagged before reading the keys only a used font has

[jakejackson1]

Summary

A follow-on from #18, which found this defect in the twin code path but left it alone as out of scope.

BackgroundWriter::writePatterns() writes a resource dictionary for an SVG used as a repeating background, and picks the fonts for it out of $this->mpdf->fonts:

foreach ($this->mpdf->fonts as $font) {
	if (!$font['used'] && $font['type'] === 'TTF') {
		continue;
	}
	if (isset($font['fo']) && $font['fo']) {

used and type are read before fo — but a core font that has not drawn anything yet carries neither key. So the loop raises Undefined array key "used" on PHP 8 for any page whose only text lives inside the SVG background, which is precisely the situation in which this loop runs at all.

Testing the flag first is enough. This is not a mirror of anything upstream; the same shape landed in FormWriter in #18, where the identical order broke Issue539Test and Issue990Test.

Try it

$mpdf = new \Mpdf\Mpdf(['mode' => 'c']);
$mpdf->WriteHTML(
	'<div style="background-image: url(\'tests/data/img/pattern-with-text.svg\');'
	. ' background-repeat: repeat; height: 40mm"></div>'
);
$mpdf->Output();

On gravitypdf, with display_errors on:

Warning: Undefined array key "used" in src/Writer/BackgroundWriter.php on line 166

On this branch, nothing. Note the <div> has no text of its own — that is what leaves the core font registered but unused.

Test plan

• composer test — 1050 tests, 2533 assertions, green (gravitypdf is at 1048).
• composer cs — clean.
• PHPStan — same 33 findings as gravitypdf, none new.
• tests/Mpdf/BackgroundPatternResourcesTest.php — both cases error on gravitypdf, pass here.
• The generated PDF is byte-identical to gravitypdf's once the creation date is normalised, so this is the warning and nothing else.

More info

Behaviour is unchanged

Reordering the guards cannot change which fonts are listed:

Font	Before	After
no fo	skipped (or warned, on the way to being skipped)	skipped
fo, TTF, used false	skipped by the first check	skipped by the inner check
fo, TTF, used true, SIP/SMP	one entry per subset id	same
anything else with fo	one entry	same

type is now isset()-guarded too, matching how ResourceWriter::writeResources() reads the same array.

The test

Two cases, both rendering the same page:

• /PatternType 1 is in the output, so the tiling-pattern branch really ran — without that, a passing test would prove nothing, since PHPUnit's warning-to-error conversion is what catches the regression.
• The pattern's /Resources object still names the font its Form XObject draws with, so the reorder has not quietly dropped anything. The fixture SVG carries a <text> element for that reason; none of the existing SVG fixtures do.

Relationship to #18

Independent — different file, no conflict, either order. #18 fixes the same defect in FormWriter::getResources(), which is new code there; this one has been in BackgroundWriter all along.