[2/2] gdiplus: Implement GdipImageGetFrameCount and GdipImageSelectActiveFrame.

[Vincent Povirk]

>> Er, you removed the FIXME, but GdipImageSelectActiveFrame is still a
>> stub? You're not changing the image data.
>
> GdipImageSelectActiveFrame is now implemented, the bug is elsewhere.
> Anyway, I'm working on that area on both sides of the problem.

SelectActiveFrame needs to discard the data in the current frame, and
replace it with the original data from the input file (unless the
active frame number isn't changing). We have tests
(test_multiframegif) that show that, and I intended the move_bitmap
function to also be used for this.

We know it's not just changing an active frame index because switching
to some other frame and back overwrites any changes we made to the
image data.