Y offset for 4x zoom sprites does not precisely match specification.

The 4x extra zoom sprites do not follow the specified offset requirements: Scaling of Y offset is currently simply zoom*yrel, wheras the specifications indicate it should have an additional ceil(zoom*-1/2).

This is needed for ground tiles to have perfect alignment with the half tile foundation sprite. Currently, the vertical offset is off by 2. By extension, all ground tiles for buildings etc. need the same offset adjustment, and so all buildings on them do too.

The effect is subtle, perhaps most visible for snowy rail half tiles against the normal arctic terrain background. The half tile rail sprite is not cut at precisely the widest part of the tile, and the half tile foundation does not tesselate in (the 2px wide rear wall is not visible).

Read the linked RealSprites zoom-level specification, then locate the 4x zoom sprite Y-offset scaling described in the issue. Verify the adjustment against ground tiles and half-tile foundation sprites; it is done when the vertical offset follows the specification and the affected tiles align precisely.