Add support for two different 24-bit colour methods.

1) XTerm using semicolons.  CSI 38/48 ; 2 ; 0 ; R ; G ; B m
2) PabloDraw/TundraDraw. CSI 0/1 ; R ; G ; B m

These use a temporary internal palette of 15840 entries to ensure what's
on the screen is correct.  Scrollback data may be incorrect however.