mp4: Eliminate duplication between the two open functions.

mp4: Eliminate duplication between the two open functions.

The only difference between mp4_open_read() and mp4_open_meta()
is that they pass different values for the meta_only flag to
parse_root_atoms(). We can avoid some duplication by moving the
common code to parse_root_atoms(). Rename that function to open_file()
because it now does more than just parsing atoms.

The patch also changes the prototype of both public open functions
to return an integer error code in addition to the pointer to an mp4
structure. This allows us to gradually improve the error diagnostics.