fix Gaussian mipmap generation in FSLoader:

fix Gaussian mipmap generation in FSLoader:
* use the correct sigma value for downsampling (its sqrt(0.75), not 0.75)
* use ImageJ GaussianBlur instead ImageFilter, faster thanks
  multi-threading and scaling (for larger sigmas)
* use accurate 1/2 resizing as introudced in the last commit
* remove clutter in the GRAY8 branch and use the available methods instead
* fixed comment that said jpeg even if it was tif or png
* added a comment to the AWT based part (that one is not fixed regarding
  accuracy nor the final rendering)

TODO Evaluate the AWT speed gain relative to util.Downsampler, particularly
when not doing pixel-size based scaling!  If either of those methods would
be used at some point, the rendering of images must be aware of that and
compensate for pixel offset differntly than for the Gaussian method.