game: texturing fixed

* repeated textures are now stored
  separately, i.e. not in atlas, this is the only sane way
  to do texture repeat in OpenGL. Also, we use trivial pixel shader
  now
* texture atlas now has 2 pixel padding and atlas texture coordinates
  are now inflated by 0.5 (instead of being deflated by 0.5), this
  ensures correct bilinear sampling
* textures now support setting wrap mode, this is essential for things
  like grass, when x wrap mode must be "repeat" and y wrap mode must
  be "clamp", otherwise artifacts at top and bottom of the grass are
  inavoidable (they happen due to linear sampling in y direction)
* using GL_LINEAR_MIPMAP_NEAREST instead of GL_LINEAR for
  minification filter, this yields much better picture when we zoom
  the scene out