Fix imagery artifacts on a number of systems

After commit 8f47354872b0aa76031fd02f7111beb2d338ba5d, we started seeing small cracks between imagery tiles on the 2010 MacBook Pro on my desk when forcing integrated (Intel) graphics, and on @shunter's Mac Mini with an AMD GPU.  Then, a user reported some major imagery artifacts (https://groups.google.com/d/msg/cesium-dev/e29qwfUNkyk/E3q_LK-HvjwJ), traced to the same commit, on a Windows 7 system with a very recent NVidia GPU.

This pull request reverts a portion of that commit, which fixes the problem on the two Macs.

It is totally baffling to me that the original change would cause artifacts.  It seems like some kind of sketchy interaction between mipmaps (the problem goes away if I disable them) and a conditional in the fragment shader, but it shows up on GPUs from at least two and possibly three manufacturers.  @pjcozzi or anyone else, if you have any ideas, I want to hear it.  In the meantime, this change is low-impact and we should get it out there in case other people are having problems as well.