Make the cursor colour override true colour.

Otherwise, moving the cursor (at least in active, filled-cell mode) on
to a true-coloured character cell causes it to vanish completely
because the cell's colours override the thing that differentiates the
cursor.

unix/gtkwin.c

@@ -3274,6 +3274,7 @@ void do_text_internal(Context ctx, int x, int y, wchar_t *text, int len,
 	else if (nbg >= 256) nbg |= 1;
     }
     if ((attr & TATTR_ACTCURS) && !monochrome) {
+        truecolour.fg = truecolour.bg = optionalrgb_none;
 	nfg = 260;
 	nbg = 261;
     }