Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

16 bit is not enough for volume calculations.

Move from PcmClamp().

Allow 32 bit platforms to use 32 bit instead of 64 bit for summing 24
bit samples.