util: add new fragment shaders to simple_shaders

util: add new fragment shaders to simple_shaders

New shaders:
* Fragment shader which writes depth sampled from a texture
* Fragment shader which copies COLOR[0] to multiple render targets

Additional improvements:
* The fragment 'tex' shaders now take a sampler type (TGSI_TEXTURE_*)
  so that they can sample from any type of texture, not only from a 2D one.