utf8.c: Avoid unnecessary UTF-8 conversions

utf8.c: Avoid unnecessary UTF-8 conversions

This changes the code so that converting to UTF-8 is avoided unless
necessary.  For such inputs, the conversion back from UTF-8 is also
avoided.  The cost of doing this is that the first swatches are combined
into one that contains the values for all characters 0-255, instead of
having multiple swatches.  That means when first calculating the swatch
it calculates all 256, instead of 128 (160 on EBCDIC).

This also fixes an EBCDIC bug in which characters in this range were
being translated twice.