On Windows, GdipCreateBitmapFromHBITMAP creates a copy of the bitmap, not
a reference. Currently, we match this behavior, but this is only because
we cannot yet create bitmap objects that reference existing memory. If
GdipCreateBitmapFromScan0 were fixed to do this, FromHBITMAP would break.

Therefore, we always pass NULL to FromScan0 so that it allocates new memory
for the bitmap.