Improved WPF tile rendering

Improved rendering of tiles to hide lines between tiles.
Added IViewport.WorldToScreenUnrotated() which goes along with
IViewport.WorldToScreen(). The first converts world coordinates to
screen coordinates assuming the screen isn't rotated while the later
does a full conversion taking rotation into account. The
WorldToScreenUnrotated() is useful for placing WPF elements in cases
where a transform will handle the rotation portion of the positioning.