First step of dead pixel removal, and the one that decides whether the
rest is worth building: where the map comes from.
`rawler` is no help. It knows `OpcodeList1/2/3` exist — it copies them
through when *writing* a DNG — but it never decodes them, and the
`dng_tags` map it exposes is only ever filled by callers, never by a
decoder. So the bytes are read from the IFD directly, which this module
was already walking for previews, including the SubIFDs where a DNG keeps
its raw IFD.
`OpcodeList1` specifically: lists 2 and 3 run after demosaic and after the
colour transform, so neither can carry a correction that has to happen on
the mosaic. Two opcodes describe defects — `FixBadPixelsList`, which is
explicit coordinates plus whole dead rows and columns, and
`FixBadPixelsConstant`, which names a sentinel value rather than any
coordinates and is left unimplemented until there is a stage to consume
it. A half-implementation that guessed at coordinates would be worse than
the absence, because it would look like it worked.
Two things the tests pin down because both are silent when wrong: a point
is stored (row, column) and reading it the other way round lands the
correction on the wrong photosite — invisibly, on a square crop — and
opcode payloads are big-endian whatever the container's byte order is, so
a little-endian TIFF still writes these the other way round.
An unknown opcode is stepped over using its declared length rather than
abandoning the list, because a camera that corrected its lens as well as
its sensor writes both, and losing the map whenever a warp is present
would be losing it on most files that have one.
Includes `--example defects`, because whether any of this fires is a
question about a particular library rather than about the specification.
