dcadec: store the stereo downmix coefficients as float in the DCAContext.

The 7-bit codes previously used are absent from the ETSI 102 114 V1.4.1 spec.
dca: convert dca_default_coeffs to float.

Easier to read, modify, and avoids relying on an outdated table.
dca: remove embedded downmix coefficient extraction.

It was based on an old, seemingly incorrect specification, so default coefficients were always used anyway.
dca: extract core substream's embedded downmix coeffcient codes, if present.

As per ETSI TS 102 114 V1.4.1 specification.
dca: improve default stereo downmix coefficients.

Previous coefficients were producing a non-normalized, incorrect Lt/Rt downmix.

The new coefficients produce a standard Lo/Ro downmix with 3dB attenuation.

This should match the typical default coefficients used by DTS encoders.