remap_palette (potential) incorrect behavior if source_palette has 768 elements #9865

This is a quick potential one-character fix. But certainly longer, as it is better to write tests about it.

What happens?

I was reading this code:
(which was added in d522e0a)

And I realized this is probably buggy. Because 3 * 256 = 768. But that code only detects RGBA if len(source_palette) > 768. It should have been >= instead of >. Probably.

What did you do?

I just read the code. I did not write any code (yet). I did not use the remap_palette function. And, in fact, I do not intend to use it in the near future. I just read the code and found a potential bug. Consider it a late code review. ;)

Proposed solution?

1. Write down some tests using RGBA palette.
2. Check if the tests fail. If my understanding is correct, the tests will fail.
3. Replace > with >=. The tests should now succeed.

Can someone from Pillow please investigate and check if this is the case?

Thanks! I hope my little bug report over here helps. :)