Make TermWin's palette_get_overrides() take a Terminal *.

Less than 12 hours after 0.75 went out of the door, a user pointed out
that enabling the 'Use system colours' config option causes an
immediate NULL-dereference crash. The reason is because a chain of
calls from term_init() ends up calling back to the Windows
implementation of the palette_get_overrides() method, which responds
by trying to call functions on the static variable 'term' in window.c,
which won't be initialised until term_init() has returned.

Simple fix: palette_get_overrides() is now given a pointer to the
Terminal that it should be updating, because it can't find it out any
other way.