Fix sound volume after state restore
Fix sound volume after state restore

Several volume mixer settings are mapped to two different mixer register
addresses: one for higher precision, and a lower precision one for
legacy reasons.

Windows 98 uses the new registers and leaves the legacy registers
untouched (which have a value of zero), but when v86 restores from a
state file, the old legacy registers are being re-written to and
incorrectly overwrites the new register values with zero.

mixer_full_update now skips over legacy register values. The new, higher
precision registers are the single source of truth and legacy register
values should be calculated from the new registers.

Fixes the Windows 98 sound demo.

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