[2/2] gdiplus: Add preliminary support for pixel offset modes.

[Vincent Povirk]

>          return sample_bitmap_pixel(src_rect, bits, width, height,
> -            gdip_round(point->X), gdip_round(point->Y), attributes);
> +            point->X + pixel_offset, point->Y + pixel_offset, attributes);
> +    }

This is going to break tiling. By removing gdip_round you're
implicitly truncating the number instead of flooring, which will give
you the wrong result when X or Y is negative. I suggest explicitly
adding a floorf.