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

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

Makes the coefficients easier to read and/or modify.
dca: improve defaultstereo downmix coefficients.

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

The new coefficients produce a standard Lo/Ro downmix (non-normalized but with 3dB attenuation).

This should match the typical default coefficients used by DTS encoders.
dca: remove embedded stereo downmix coefficient extraction.

It was based on an old, incorrect specification, so default coefficients were always used anyway.
WIP #1/2.