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.

Negating an unsigned integer does not work.

For type safety and code readability.

Avoid roundtrips to floating point.

This check was redundant, because we could only exit the loop when
ret==DECODE_OK.

Not necessary.

Reduce header dependencies.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.