I am trying to add a pdf compression level parameter to the rc file,
with possible values 0 (no compression) or 1 to 9 (as in gzip). If I
make the default nonzero but set the value to 0 in my rc file, the
default takes over. It seems to me that the two "if cval" tests in
rc_params should be changed to "if cval is not None"; but would this
have unintended consequences?