allocate hIGFEnc handle only when needed

• Related issues: #157 (closed)
• Requested reviewers: @vaclav
• Link to long BE test: https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/jobs/153541

Reason why this change is needed

Significant memory saving in IGF part.

Description of the change

hIGFEnc handle is only allocated when really needed.

Affected operating points

Only some operating points with IGF are affected, but no difference in bitstream and output.

• Bitstream compatibility: 1. Produced bitstream is BE compared to previous state.
• Bitexactness: BE