Heretic and Hexen sky seam bug

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In vanilla Heretic and Hexen, the sky texture will display a visible seam when tiled vertically. Normally, this is hidden by the level's walls, but if the construction of a map allows the player to see the bottom tiling point of the sky, a pair of horizontal lines appear. This is due to the game assuming that sky textures must be exactly 200 pixels tall and rendering non-pixel patch data as if it was pixel data.

Certain patch import techniques, notably that used in versions of SLADE post v3.1.1.5, can result in graphical data that places the seam at a point where it is always visible.