Revision r2141
Upload sky textures with s wrapping set to GL_CLAMP{_TO_EDGE}.
This fixes the vertical (~1 texel wide) stripes that are especially
noticable with tiled art skies and Polymost with r_parallaxskyclamp=1
or Polymer (which always uploads skies with repeating s and t coords).
What constitutes a sky tile is currently hardcoded to be >=78 and <=99.
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