shadow volumes broken in master at near distances

[likangning93]

Shadow volumes (underpinnings of all the polygon/corridor stuff that's clamped to ground) seem to be broken at distances close to the camera. It kind of looks like the camera's near plane is causing problems here. Steps to reproduce:

1. launch the Ground Clamping Sandcastle in master
2. select Draw Rectangle
3. go to a corner and zoom in to a view something like this:

@bagnell do you think this might be related to changes in #5851?

[hpinkos]

For reference, here's what the problem looks like

@bagnell we should fix this before the next release

[likangning93]

Setting viewer.scene.logarithmicDepthBuffer = false; gets rid of the problem, so more evidence to this being a log depth thing.

[likangning93]

@bagnell I think this is related to this comment: https://github.com/AnalyticalGraphicsInc/cesium/pull/5851/files#r172639411

Forcing DISABLE_GL_POSITION_LOG_DEPTH hereabouts seems to help for me. I'll investigate a bit more and open a PR, but I'm... not super confident heheh.