Add macro to get the file descriptor out of an FD object

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>
v4l2src: Add mutex to protect parts of the buffer pool

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>
v4l2src: replace raw FD with FD object

This is necessary since the buffer pool may outlive the src

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>