Apply opacity and transform to blocks, tables and inline elements, not only images #602

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

opacity and transform are read on images only. On blocks, tables, cells and inline elements they are ignored. Templates use opacity for faded "PAID" or "DRAFT" stamps and muted text, and transform: rotate() for stamps and angled labels.

Measured on gravitypdf at 60b82630, PHP 8.5.4, from the ExtGState objects and cm operators written.

Cause.

• Img::open() reads OPACITY (src/Tag/Img.php:143-145) and TRANSFORM (:338-339) into the image's object attributes.
• printobjectbuffer() applies them when it draws the image (src/Mpdf.php:8120-8130), using transformRotate(), transformScale() and the other transform helpers (:27774 onwards).
• setCSS() and BlockTag, Table and Td never read either property. mPDF's own rotate on positioned blocks and tables is separate.

Fix.

• opacity on a block, cell or table.
  • CSS applies it to the element and its content as one group, so overlapping children do not show through each other. In PDF that is a transparency group: draw the element's output for each page into a Form XObject with /Group << /S /Transparency >>, then paint it with /ca and /CA.
  • Setting /ca on each piece instead is simpler, but wrong where a background and text overlap: the background shows through the text.
  • A block that crosses a page needs a group per page.
• opacity on an inline element. Text runs have nothing to overlap but themselves, so SetAlpha() (src/Mpdf.php:2432) around each run of the element is close enough.
• transform on a block or table.
  • Transforms do not affect layout. Lay the box out as normal, then wrap everything it drew on the page in q <matrix> cm … Q.
  • transform-origin defaults to the centre of the border box, whose height is known only at the close. Insert the q … cm at the block's start in the page's content when it closes, as mPDF does for backgrounds (___BACKGROUND___PATTERNS).
  • Decide what a transformed block does at a page break: transform each fragment, or keep the block whole as page-break-inside: avoid does.
  • Link and annotation rectangles are axis-aligned in PDF. Transform the corners, and use the bounding box for anything rotated.
  • Accept the same functions as images, and transform-origin.
• Standards. PDF/A-1 and PDF/X-1a forbid transparency. Follow the transparencyAllowed() path box shadows use: warn, and draw opaque.

Tests. opacity on a block with a background and overlapping text, on a nested block with its own opacity (the values multiply), on a block across a page break, on a span, and on a cell. rotate, scale, translate and a list of transforms on a block, with transform-origin. A rotated block holding a link. opacity under PDF/A-1b.