Convert most unchecked size/index `as` casts to checked casts with
`TryFrom`, included another top-level error variant.
This has several motivations:
- Making the library as robust as possible, including to platform
differences.
- Unchecked casts are susceptible to breakage during refactorings.
- Preparing for BigTIFF support, which will require the introduction of
generics and associated types for some parts of the de/encoders.
Unchecked primitive casts cannot be bound, while TryFrom can, and
overflowing conversions become more likely.
- Remaining u16 casts may be prone in failure in existing practice and
not just in theory during future changes.
Tag representations are not decoder- or encoder-specific, so should live
in their own module accessible to both for reuse. The decoder::ifd
module remains since this module is mainly dedicated to decoding the
IFD.
Resolve several TODOs where tag values were cast from u32 without checks
by providing generic methods that error when the value fails casting to
the desired type. In addition to being more correct, this simplifies and
shortens decoding logic.
Remove `find_tag_u32` and `find_tag_u32_vec` as they are now redundant
and a GitHub code search found no usages outside copies of
`image-tiff`.
Make the behavior of RGB and RGBA like Gray in returning ColorTypes for
all homogeneous bit depths, even if those depths are not 8, 16, 32 or
64. Note that this does not change which color types can actually be
decoded.
Note that this adds the LONG8 data type from BigTIFF because of bounds
requiring TiffValue. However, this datatype should not actually be
written to tags.
Andrew Champion