Render to texture Skybox alpha fix.

When rendering the game too a texture, for future post-processing work or Oculus Rift, the billboards were being rendered incorrectly against the background. The reason for this was that it was passing through the alpha value from the underlying texture directly.

I've now fixed this to be just the colour from the skybox texture and a fixed alpha value of 1.0

This commit re-enables the Render-to-texture so maybe one day we can have [nice shiny things](http://pioneerspacesim.net/forum/viewtopic.php?f=3&t=14#p91).