fix(pdf): keep the soft mask of a JPEG image

[andiwand]

Refs #1188, refs opendocument-app/OpenDocument.droid#703

A user reports that a transparent image in a PDF shows with a black background. The report has no sample file, but one cause reproduces on current main: encode_image passes a DCTDecode JPEG through to the browser undecoded and drops its /SMask or stencil /Mask. Producers often store a transparent image as a JPEG with a separate soft mask, and the JPEG holds black under the transparent pixels.

The common case already works: LibreOffice writes a transparent PNG as Flate with an /SMask, and odr renders that correctly.

Before and after

A probe PDF draws a red rectangle, then a JPEG of a blue circle on black with a Flate /SMask that is opaque only inside the circle.

• Ghostscript: the blue circle on the red rectangle.
• main: a black square with the circle in it.
• this PR: the blue circle on the red rectangle, as in Ghostscript.

Change

• encode_image keeps the JPEG as it is, and writes its alpha plane as an RGBA png (EncodedImage::alpha_png). XObject::image_alpha and ImageElement::alpha_mask carry it to the renderer.
• The pdf view writes the png as a <mask mask-type="alpha"> in the image's own unit square, one for each mask on a page (MaskRegistry::register_image_mask), and the <image> references it. The browser composites. No JPEG decoder is needed. The alpha soft masks of the graphics state already use mask-type="alpha", so this needs no browser support that the pdf view did not need before.

Limits

• A colour-key /Mask on a JPEG still needs the decoded samples, so it stays unsupported.
• An image inside a tiling pattern is not masked, because the pattern tiles are written without the page's mask registry.

Verification

• PdfFile.a_jpeg_keeps_its_soft_mask: a JPEG with an /SMask gets one mask, and a JPEG without one gets none, in both text modes. Without the change, the test fails.
• The PDF and html tests pass (471 tests).

The report has no sample, and OpenDocument.droid 4.21.0 still uses core 7.4.0. So I suggest closing #1188 only when the reporter confirms the fix, or sends a file.

Reference output

PDF pages with JPEG images that have soft masks will change. I will update the reference output after this PR and #1200 have landed.