[wleeaf]

What kind of change does this PR introduce?

Bug fix. A PNG image with a palette of 1, 2 or 4 bits per index and a tRNS chunk comes out mostly transparent. tests/images/pallete-transparent-white-1bit.png is an example: drawn with doc.image(), only about 4,300 of its 30,400 dark pixels show up when the page is rendered at the image's size, and the rest of the image is blank.

loadIndexedAlphaChannel reads the pixels png-js decodes as one palette index per byte. For fewer than 8 bits per index, png-js leaves several indices packed in each byte, so each pixel gets the alpha of a byte made up of several indices. The alpha channel also runs out of data after width * height * bits / 8 values, and the pixels after that stay transparent. The image data itself is fine, because it goes into the PDF as packed PNG rows with a PNG predictor.

png-js's decodePixels can't simply be reused for these images: it unfilters rows in steps of bits / 8 bytes, a fraction of a byte, whereas PNG filters them a byte at a time. So for non-interlaced images with fewer than 8 bits per index, this change inflates and unfilters the rows itself and unpacks the indices. It adds inflateSync to the zlib wrappers for that, using unzlibSync from fflate in the browser build. Images with 8 bits per index, and interlaced ones, go through png-js as before.

The rendering of the fixture above now matches its decoding by Pillow. With PNGs I made with 4 and 2 bits per index and one transparent entry, the mean difference per channel drops from 117 and 77 to 0.1 and 0.4.

Checklist:

• Unit Tests: Pallete indexed transparency 1bit alpha channel in tests/unit/png.spec.js runs the real loadIndexedAlphaChannel on the 1-bit fixture. It checks that there is one alpha value per pixel and that 6,900 pixels are transparent and 7,600 opaque, as Pillow decodes the image. On master it finds 12,737 transparent pixels.
• Documentation N/A
• Update CHANGELOG.md
• Ready to be merged

npx vitest run tests/unit (525 tests) and tests/visual (53 tests) pass, and eslint and prettier are clean on the changed files.