text: GL_SGIX_depth_texture
  http://www.opengl.org/registry/specs/SGIX/depth_texture.txt
  GL_SGIX_depth_texture
  	GL_DEPTH_COMPONENT16_SGIX 0x81A5
  	GL_DEPTH_COMPONENT24_SGIX 0x81A6
  	GL_DEPTH_COMPONENT32_SGIX 0x81A7
  