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>

include/libcamera/meson.build

@@ -8,6 +8,7 @@ libcamera_public_headers = files([
     'controls.h',
     'event_dispatcher.h',
     'event_notifier.h',
+    'extensible.h',
     'file_descriptor.h',
     'framebuffer_allocator.h',
     'geometry.h',