fix #3715: make all info-textures individually queryable by Lua (gl.Texture)

notes:
  "$info" and "$extra" still map to the active info-texture as before
  "$info_losmap" or "$extra_losmap" map to the  L-view texture
  "$info_mtlmap" or "$extra_mtlmap" map to the F4-view texture
  "$info_hgtmap" or "$extra_hgtmap" map to the F1-view texture
  "$info_blkmap" or "$extra_blkmap" map to the F2-view texture

  textures contain no data until user switches to their respective modes
  non-active textures are not updated in the background, so will go stale
  (might be better to automatically delete them after N frames, undecided)