When trying to figure out the correct implementation, we now have a very
strong distinction on plugins that are well suited for sniffing, and
plugins that need a MIME type to be chosen.
Instead of having multiple calls to non-static virtual sniff methods for
each Image decoding plugin, we have 2 static methods for each
implementation:
1. The sniff method, which in contrast to the old method, gets a
ReadonlyBytes parameter and ensures we can figure out the result
with zero heap allocations for most implementations.
2. The create method, which just creates a new instance so we don't
expose the constructor to everyone anymore.
In addition to that, we have a new virtual method called initialize,
which has a per-implementation initialization pattern to actually ensure
each implementation can construct a decoder object, and then have a
correct context being applied to it for the actual decoding.
Error::from_string_literal now takes direct char const*s, while
Error::from_string_view does what Error::from_string_literal used to do:
taking StringViews. This change will remove the need to insert `sv`
after error strings when returning string literal errors once
StringView(char const*) is removed.
No functional changes.
PBM, PGM, and PPM image loaders are mostly common. The only difference
is how the data is read and the associated magic numbers. The magic
numbers are already made common using the loading contexts. Now make
the implementations common via a class template which accepts the
context to disambiguate.
Much of the code in PBM, PGM, and PPM image loaders is common. The
contexts are nearly identical. Instead of writing multiple contexts,
write 1 with a template argument to pass in the details of the given
format.
