LibGfx/GIF: Correctly write frames with a non-null position

[LucasChollet]

Fix #24411

[nico]

Maybe tweak the animation test to hit this case (see webp animation test for an example).

[LucasChollet]

Sure, you'll have to wait until tomorrow for that tho

[LucasChollet]

see webp animation test for an example

This was the worst advice you could give me. I copied some parts of it, including this line, which has a behavior that isn't really intuitive:
rgb_bitmap_2->scanline(3)[3] = Color::Red;
(hint: type of the lhs is ARGB aka int)

Fun fact: for some reason that I don't understand, the WebP test fails if you write a color with an alpha value equal to 255, but pass otherwise. I'll let you fix this one. See #24428

[nico]

#24433