Rich text files have groupings of text, where styles are pushed onto
the stack when encountering a start of the group, then popped at the
end of the group.  This was being handled improperly before, because a
single styleChanged flag was being stored to keep track of whether the
style needed to be restored at the end of a group. This fails to work
properly since the single flag isn't keeping track of all the levels
of the stack, so some styles are not restored properly.

When a colour table entry is empty, then the default colour is used.
For an incomplete colour table entry 0 is used for the missing colours.

Previously the -1 value used internally for missing colours was being
converted into white, where it should be using the default colour that
is normally black.

This bug could be seen by loading the following rich text into wordpad:
{\rtf{\colortbl;;}\cf1 text}