libcamera: v4l2_videodevice: Reintroduce toV4L2PixelFormat()

This is a partial revert of commit 395d43d6d7 ("libcamera:
v4l2_videodevice: Drop toV4L2PixelFormat()")

The function was removed because it incorrectly maps non-contiguous V4L2
format variants (ie V4L2_PIX_FMT_YUV420M) to the API version supported
by the video device (singleplanar API and multiplanar API).  It was
decided at the time to remove the function and let its users call
directly V4L2PixelFormat::fromPixelFormat() which accepts a
'multiplanar' flags.

As we aim to associate multiple V4L2PixelFormat to a single libcamera
format, the next patches will verify which of them is actually supported
by the video device. For now, return the contiguous version
unconditionally.

Re-introduce V4L2VideoDevice::toV4L2PixelFormat() and convert all
the V4L2PixelFormat::fromPixelFormat() users to use it.

The V4L2 compatibility layer is the only outlier as it doesn't have a
video device to poke, hence it still uses
V4L2PixelFormat::fromPixelFormat().

Next patches will implement the device format matching logic and handle
the non-contiguous plane issue in V4L2VideoDevice::toV4L2PixelFormat().

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Paul Elder <paul.elder@ideasonboard.com>

include/libcamera/internal/v4l2_videodevice.h

@@ -228,6 +228,8 @@ public:
 	static std::unique_ptr<V4L2VideoDevice>
 	fromEntityName(const MediaDevice *media, const std::string &entity);
 
+	V4L2PixelFormat toV4L2PixelFormat(const PixelFormat &pixelFormat) const;
+
 protected:
 	std::string logPrefix() const override;