gdiplus: When copying an image do not round pixel positions up.

[Vincent Povirk]

> In my tests (which replicate the behaviour of a target application) if
> an image of say 10x10 pixels is being copied with a magnification world
> transform then the destination image has right/bottom border completely
> wrong, and my patch fixes this. Besides, bilinear interpolation does
> exactly that already - doesn't round pixel positions up, which contradicts
> your explanation.

The same rule about pixel centers applies with bilinear interpolation.
That means that at the lower-right corner of a stretched image you end
up with places where the lower-right pixel was blended with a pixel
from outside the image.