The WindowExtent was represented by a BoundingBox which needs to be
axis-aligned since it only stores min and max points. If the viewport is
rotated, the actually visible extents (WindowExtent) will not be
axis-aligned. Added the Quad class which stores 4 corner points that do
not have to be axis-aligned and used this class to represent the
Viewport.WindowExtent instead of BoundingBox so it would contain the
proper values when rotated.

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.