Combining these two in one single value saves some memory, but is
complicated and we may need the src_sample_format for new features
later.

Similar to commit 32380d1d, these are
the final parts for really fixing
https://github.com/MusicPlayerDaemon/MPD/issues/469

These options are exclusive.

First part of the "real" fix for
https://github.com/MusicPlayerDaemon/MPD/issues/469

Preparing to add more state.

The dop_buffer will be moved out soon.

Reduce header dependencies.

Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.

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.

The compile-time calculation for `factor` overflows because `1<<31`
cannot be represented by `int`.  By casting to `uintmax_t` first, we
can avoid this overflow.

Closes #380

This assertion currently fails for S32 due to integer overflow (#380).

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.