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This commit ensures that when a series of messages read from
either the compositor or application is truncated halfway through
a message, that the truncated message is completed and correctly
parsed on the next iteration of the message transport state machine.

Previously, the main proxy loops in `run_server_child` and
`run_client_child` alternated between making transfers from their shared
channel to their individual wayland compositor/application channels.

By using socket connections in nonblocking mode, the proxy now can
simultaneously maintain transfers in both directions. This ensures that
large transfers (such as a full-screen image) do not prevent e.g. key
release events from being transferred.

Additionally, as the two proxy loops were very similar, they have been
 combined into the `main_interface_loop` routine.

The code still does not correctly handle the case where a read from
the wayland compositor/application fills the entire read buffer and
truncates a sent message, but it is now easier to adjust for that case.

When shadow_fd structs _that are owned by known protocol objects_
have their reference count drop to zero, they are deleted.

Newly created shadow_fd's begin with a reference count of 1,
which is reduced when the translated fd to sent to the
display/application/other waypipe socket end. This ensures
that the object is not deleted early, even if the compositor
e.g. wl_display.delete_id's the fd owning object before receiving
it.

For wl_shm_pool.resize, wl_shm.create_pool, and wl_surface.commit .

* Make scdoc optional; documentation is not strictly necessary
* Explicitly test for fmacro-prefix-map support
* Test for the minimum possible versions of wayland dependencies
* Reduce required meson version

Every wp_object has a nullable reference to the shadow structure
for a file descriptor. As the existence of wayland protocol
objects is not guaranteed in the face of compositor/application
misbehavior, when e.g. a wl_buffer is created from a wl_shm_pool,
the wl_buffer registers a reference to the file descriptor that
the wl_shm_pool had. Methods that imply a file descriptor change
(wl_keyboard.keymap, wl_surface.commit) mark the object's referenced
file descriptor as dirty, so that changes can be transferred.

This commit does not yet implement any deletion logic.

No real content yet. Using clang-format as the least of all evils.