Add palette and 1-bit grayscale PNG support

[rowland]

Summary

• Expand PNG decoding to accept color type 3 palette images at 1/2/4/8-bit depths and normalize them into the existing PDF image path.
• Allow low-bit grayscale PNGs, including 1-bit grayscale, and standardize decoded sample data to 8-bit output.
• Preserve palette transparency via tRNS by keeping the soft-mask flow, and drop alpha metadata once a decoded image is fully opaque.
• Add focused tests covering palette PNGs, palette transparency, and 1-bit grayscale PNGs with synthetic in-repo fixtures.

Testing

• go test ./pdf
• go test ./...
• go build ./...
• Smoke-tested representative external PNG assets from ~/src/re2/assets through PrintImageFile