GL_INTEL_texture_scissor
http://oss.sgi.com/projects/ogl-sample/registry/INTEL/texture_scissor.txt
void glTexScissorFuncINTEL (GLenum target, GLenum lfunc, GLenum hfunc)
void glTexScissorINTEL (GLenum target, GLclampf tlow, GLclampf thigh)