Found with -Wdouble-promotion
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

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.

Don't require a quality or bitrate setting.  If nothing is set, don't
fail startup - just go with a good default.  A quality setting of 3 is
what "oggenc" defaults to as well.

The destructor will call this in any case.

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

The old struct config_param remains only for top-level string options.

Replaces g_ascii_strup() and allows building the Vorbis encoder
without GLib.

Enables using range-based "for".

.. and move the plugins to src/encoder/plugins/.