V4l2.get_fd: added an accessor for the file descriptor
v4l2_get_fd: fixed to use Val_int, not Int_val
V4l2.t#rgb_ba, #raw_ba: added methods for accessing data with lower overhead
V4l2: array_frame and rgb_array_frame now use 'int', not 'char' as the OCaml data type
V4l2.get_frame: reduced code duplication