Frustum experiment

[loshjawrence]

For the log depth / multi frustum stuff, here's another idea for frustum/projection. But I guess there are too many things like putting 3dtiles on top of terrain that make this more complicated that what I wrote...

• Use a projection that maps near far to 1-0 so that all the precision is in the back of the frustum.
• Do a z-prepass and find min max of the depth buffer to use for a tight near far. See section 4 of http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.360.3911&rep=rep1&type=pdf
• post process the depth buffer to remap the gl_FragDepth values assuming the tight near far (may have to use a polygon offset in one of the writes to be on the safe side, i.e. push them back a little assuming the next step...)
• Proceed as normal using the depth buffer to get some early z since its a decent/conservative approx of depth (still write actual depth values).

[OmarShehata]

@loshjawrence if I understand the context here, these are some of your suggestions for fixing a particular bug. If so, can you put these notes in the relevant issue and close this? Otherwise this is just going to get lost.

[loshjawrence]

I don't think this should be considered among fixes for the code sprint since it could end up being a considerable amount of work and end up not fixing anything, just something to experiment with in the future.