Detecting color depth-16 color, 256 color, or 32 bit color

[SMcNeill]

Set a flag.  That checks for 16 NON-BLACK colors.

If theres a black in those first palette entries, and then 16 other colors, that's NOT a 16-color palette.

For example:

Black
Red
Blue
Green
White
Black * 250

The above would count as 4 non-black colors, but there's a valid Black in the 5-color palette.  I wouldn't call that a 4-color image.

The <= 16 check is for:
  standard 16 color palette (black + 15 others)
OR
  non-black palette (someone swapped black for midnight blue.)

But it's going to glitch with:
  Black + 16 other colors, which is actually a 17-color palette and probably a 256 color image.

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Subtle glitch, but one that a library function should account for.  You never know who or where a library ends up being exposed to.