Changing the colour of the second pixel of the source bitmap clarifies
the incorrect behaviour of PixelOffsetMode Half and HighQuality.
This also provides better proof that PixelOffsetMode None and Fast are
implemented correctly.
Extending tests is starting point for further
improvements of implementation pixel offset modes
to match implementation from native gdiplus.
--
v2: gdiplus/tests: Extend DrawImage scaling tests.
https://gitlab.winehq.org/wine/wine/-/merge_requests/3774