libcamera: request: Make Request class Extensible

Implement the D-Pointer design pattern in the Request class to allow
changing internal data without affecting the public ABI.

Move the internal fields that are not needed to implement the public
API to the Request::Private class already. This allows to remove
the friend class declaration for the PipelineHandler class, which can
now use the Reques/* SPDX-License-Identifier: GPL-2.0-or-later */
/*
 * Copyright (C) 2022, Ideas on Board Oy
 *
 * sdl_texture_yuv.cpp - SDL YUV Textures
 */

#include "sdl_texture_yuv.h"

using namespace libcamera;

#if SDL_VERSION_ATLEAST(2, 0, 16)
SDLTextureNV12::SDLTextureNV12(const SDL_Rect &rect, unsigned int stride)
	: