Convert volume API from integers [0-128] to floats [0-1]

[Starbuck5]

In a lot of places this actually makes the code simpler. Volumes can be multiplied without then dividing by the integer range, the new SDL3 mixing API can be used without another divide into float-land, it makes one or two music codecs more straightforward.

Closes #652

A couple things I wanted to highlight in review:

• The handling of gain in music_gme.c might not be 100% the same, I'm finding it difficult to think through the x * y vs floor(x * y + 0.5)/128.
• The Timidity_SetVolume function previously took in values between 0 and 128, but compares to a MAX AMPLIFICATION of 800, and checks if the value is at least 0. That is strange.
• Is my handling of master_volume in mixer.c okay? It previously was an SDL_AtomicInt, but there is no SDL_AtomicFloat so I gave it a spinlock instead.

[Starbuck5]

Coming back to this, I see it will be overcome-by-events by icculus's SDL_remixer, where volume is controlled by a *float* gain. So I will close this out.