Use highp sampler2D when using float textures for tables

[likangning93]

#6739 observed that Cesium's float batch tables didn't seem to be working at high enough precision on some devices.
From https://groups.google.com/forum/#!topic/webgl-dev-list/aFmFGeyRVRQ, devices are free to select the precision of sampler2D when it isn't explicitly set.
We should update our batch table code to use uniform highp sampler2D when backing the batch table with float textures.

As part of this, please roll back changes from #7421 and test on a device that shows red on https://likangning93.github.io/webgl-examples/tests/floatTexturePrecision/ but no red on https://likangning93.github.io/webgl-examples/tests/floatTexturePrecisionHighpSampler/.

Good candidates as of this writing are the iPhone 7 or newer and the first generation iPad Pro or newer.