PNGGIF::Pixel
A single RGBA pixel. Channels are 0..255; a is opacity (255 = opaque).
Channels are stored as UInt8 (4 bytes/pixel instead of 4×Int32 = 16) so
megapixel Bitmaps and the cellmap downscale / composite loops touch 4×
less memory and stay cache-resident. Accessors keep the public Int32
(0..255) contract. Out-of-range values wrap (to_u8!); internal producers
already emit 0..255.