libcamera: Add a base class to implement the d-pointer design pattern

The d-pointer design patterns helps creating public classes that can be
extended without breaking their ABI. To facilitate usage of the pattern
in libcamera, create a base Extensible class with associated macros.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/meson.build

@@ -17,6 +17,7 @@ libcamera_sources = files([
     'event_dispatcher.cpp',
     'event_dispatcher_poll.cpp',
     'event_notifier.cpp',
+    'extensible.cpp',
     'file.cpp',
     'file_descriptor.cpp',
     'formats.cpp',