[Slayer] Distance field shaders are computed only after Text element becomes visible

If scene has no Text elements visible at presentation loadtime (eyeball = false in UIP file), and Text is later set to visible through setAttribute or Datainput, distance field shader computation creates a large delay. This does not happen if the presentation has any visible Text elements at load, because shaders are computed before the first frame is shown.

Text shaders should be computed and cached regardless of Text element visibility in UIP file.