Hidden face removal

1. Allow the build tool Alpha slider all the down to full transparancy when in "Select Texture" mode.
   
2. Cull those faces from graphics rendering completely (if this is not already happening).
   

The idea being that if faces hidden inside builds could easily be made tranparant (without requiring alpha textures or script), builders could optimise their builds for increased FPS.

I understand why Alpha cannot be set to 0.0 for complete objects (in the build tools), hence allowing this in "Select Texture" only.

Slider alpha is preferred to alpha textures here as the render engine has no idea which textures are totally tranparant, whereas Alpha==0.0 is a quick-enough filter decision.

This might break a lot of current effects, cel-shading comes to mind depending on what method was used to make the outer surface transparent. However, it might be useful to have a separate mechanism where you can select a face and say "This doesn't exist." The true reduction in polys might end up being pretty small though.

AFAIK cel-shading techniques (although I only know two) rely on back-face culling - revealing non-tranparant faces that would have been hidden. What I am proposing should have no effect on exisiting builds of any form as it only removes faces that would have been drawn transparant anyway.

Reduction in polys will vary from build to build. A twisted torus inside some hoochie hair might only lose 2 faces out of a few hundred. A single prim vendor could lose 1 out of 6 faces - or 2 of 6 if it's up against a wall. Two-prim vendor maybe 6 of 20. Most building walls could lose 4 of 6, a 2/3 saving.

The potential payback, for what I am hoping (almost expecting) to be a fairly small code change, should be well worth it.