Draw every layer of a multiple background #609

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

A background or background-image with several comma-separated layers draws only one of them. The layer-specific longhands (background-repeat: no-repeat, repeat-x, and so on) are read as one value. Layered backgrounds are common for a watermark or logo over a gradient, and for a corner image over a fill.

Measured on gravitypdf at 60b82630, PHP 8.5.4, on a 60 × 30 mm block, from the image XObjects, patterns and shadings written.

Cause.

• NormalizeProperties::parseCssBackground() returns one colour, one image, one repeat and one position (src/Css/NormalizeProperties.php:395-445). A gradient anywhere in the value wins, because it is tested first with a greedy (linear|radial)-gradient\(.*\). Otherwise the first url() is taken, with everything after it passed on as repeat and position.
• BACKGROUND-IMAGE is read the same way (:789-800).
• SetBackground() (src/Mpdf.php:2785) and the pageBackgrounds list it feeds (:18654 onwards) hold one image per box.

Fix.

• Split background, background-image, -repeat, -position, -size, -origin and -clip on commas outside parentheses, so rgb(…) and gradient arguments stay whole.
• Store the layers as a list. The colour comes from the last layer only.
• In CSS, a longhand list shorter than the image list repeats, and a longer one is cut short.
• SetBackground() returns one entry per layer, and the painting code draws them from last to first, under the same clip.
• Keep the single-layer path byte-identical; most documents have one layer.
• The shorthand parser's own ordering faults (a colour after url(), / size) are fixed with the other shorthand parsers before Honour !important in stylesheets and inline styles #532. Build on that parser.

Tests. Each row above, a gradient over a gradient, per-layer background-size and -position, three layers with two background-repeat values, layers on a table cell and on @page, and the snapshot suite unchanged.