.pam is a "portrable arbitrarymap" as documented at
https://netpbm.sourceforge.net/doc/pam.html
It's very similar to .pbm, .pgm, and .ppm, so this uses the
PortableImageMapLoader framework. The header is slightly different,
so this has a custom header parsing function.
Also, .pam only exixts in binary form, so the ascii form support
becomes optional.
It is unlikely this is needed anymore, and as pointed out things should
now safely return OOM if the bitmap is too large to allocate.
Also, no recently added decoders respected this limit anyway.
Fixes#20872
This enum used to store very precise state about the decoding process,
let's simplify that by only including two steps: HeaderDecoder and
BitmapDecoded.
The function signature goes from:
`bool read_number(Streamer& streamer, TValue* value)`
to
`ErrorOr<u16> read_number(Streamer& streamer)`
It allows us to, on one hand use `ErrorOr` for error propagation,
removing an out parameter in the meantime, and on the other hand remove
the useless template.
