The transparent_color field was never initialized. I _think_ this means
its value was always set to 0, or the blackest of blacks. Instead,
initialize it to the sentinel value, newly given the name
NO_TRANSPARENT_COLOR.
This exposed a second problem: The test for whether there was an existing
transparent color was wrong (backwards). I am guessing that this was not
found due to the first bug; since the converter had a transparent color,
the correct test would have led to always getting the error "Only one
color can be transparent at a time".
Closes#3723
28bd29eb42