Addition of sound sources can overflow

[BigEd]

The various sound sources should go into a saturating adder, probably - or something more sophisticated - should not overflow.

The code presently just adds the sound chip, the SID, the disk drive sample and the Music 5000, without headroom and without regard for overflow. See https://github.com/hoglet67/BeebFpga/blob/1a6c95e0968f61b66e2c7f5ae265ab96b100da2e/src/common/bbc_micro_core.vhd#L995