qv4l2: Use C library file operations with v4lconvert for raw mode devices

qv4l2 uses libv4l2 to access the V4L2 device by default, and supports a
raw open mode for direct access to the device through C library file
operations. It also uses libv4lconvert to perform format conversion, and
libv4lconvert accesses the V4L2 device through direct syscalls by
default.

Usage of raw open mode is useful to test direct access to the device,
but also to test LD_PRELOAD wrappers that intercept the C library calls,
such as the libcamera V4L2 compatibility wrapper. The usage of syscalls
in libv4lconvert prevents the latter case from working properly. Fix it
by providing file operations to libv4lconvert that use the C library
file operations when opening the device in raw mode.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

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