Optimize the case where volume is 0x8000

When left and right volume are both 0x8000, which is the most common, we can just do a memory copy which is faster (on little endian only.)

Also, we can use pushPointers on big endian, probably faster than the alternative and less code.

I have not really carefully tested perf numbers, but the sound is played properly still, and there's no reason it shouldn't be faster.  Probably not a huge gain, but hoping it will help mobile since it was showing up for @xsacha.

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