Initialize offset to zero in FT2Font::make_open_args

Clang warns that it gets a garbage value on Python 2, since
mpl_PyFile_Dup expands to a function call that does not use &offset.
It seems that the value is not used on Python 2 but it doesn't
cost much to silence the warning.
Silence some writable-string warnings

Cast string constants explicitly to (char*) since the
method calls aren't going to modify the strings.
Check the return values of mpl_PyFile_DupClose
Merge pull request #2991 from jkseppan/tidyup-ft2font

Silence some compiler warnings related to ft2font