v4l2: v4l2_compat: Add V4L2 compatibility layer

Add libcamera V4L2 compatibility layer.

This initial implementation supports the minimal set of V4L2 operations,
which allows getting, setting, and enumerating formats, and streaming
frames from a video device. Some data about the wrapped V4L2 video
device are hardcoded.

Add a build option named 'v4l2' and adjust the build system to
selectively compile the V4L2 compatibility layer.

For now we match the V4L2 device node to a libcamera camera based on a
devnum that a pipeline handler may optionally map to a libcamera
camera.

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

src/v4l2/meson.build

@@ -0,0 +1,31 @@
+v4l2_compat_sources = files([
+    'v4l2_camera.cpp',
+    'v4l2_camera_proxy.cpp',
+    'v4l2_compat.cpp',
+    'v4l2_compat_manager.cpp',
+])
+
+v4l2_compat_includes = [
+    libcamera_includes,
+    libcamera_internal_includes,
+]
+
+v4l2_compat_cpp_args = [
+    # Meson enables large file support unconditionally, which redirect file
+    # operations to 64-bit versions. This results in some symbols being
+    # renamed, for instance open() being renamed to open64(). As the V4L2
+    # adaptation wrapper needs to provide both 32-bit and 64-bit versions of
+    # file operations, disable transparent large file support.
+    '-U_FILE_OFFSET_BITS',
+    '-D_FILE_OFFSET_BITS=32',
+    '-fvisibility=hidden',
+]
+
+v4l2_compat = shared_library('v4l2-compat',
+                             v4l2_compat_sources,
+                             name_prefix : '',
+                             install : true,
+                             link_with : libcamera,
+                             include_directories : v4l2_compat_includes,
+                             dependencies : libcamera_deps,
+                             cpp_args : v4l2_compat_cpp_args)