Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

Remove another dependency on the config/Global library.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

Allocate the frame pointers only when actually decoding the file.

The old struct config_param remains only for top-level string options.