Matrix build and include cleanups

This wraps matrix.c into a Meson dependency object so it is easier to use instead of referring to ../../shared/matrix.c or whatever.

Cleanups:

• unify shared/ include directives to a single common pattern
• stop adding arbitrary directories in ad hoc fashion to header search paths
• libweston.h really should not pull in config-parser.h

WIP: Depends on !162 (merged).

See #219 (closed).