Fix transfer function texture wrap mode.

We should (almost) always use GL_CLAMP_TO_EDGE instead of GL_CLAMP.

See, e.g.,

http://www.opengl.org/wiki/Common_Mistakes#Texture_edge_color_problem
http://devmaster.net/forums/topic/4436-difference-of-gl-clamp-and-gl-clamp-to-edge/page__view__findpost__p__27580