While looking at the issues with
ixus240, I noticed that on most(?) pre-digic 6 dryos platforms, full_screen_refresh and palette_control are the same variable (the former is treated as a word and the latter as byte

), found by the sig finder using different matches.
I don't know that this is worth cleaning up, but most ports only use palette control in load_chdk_colors, where it's used like
void load_chdk_palette() {
...
palette_control = 1;
vid_bitmap_refresh();
...
void vid_bitmap_refresh() {
...
full_screen_refresh |= 3;
...
suggesting it could be eliminated completely.
thumb2 firmware use palette_control for clean overlays, and do not have full_screen_refresh