Use highp in shadow shaders which deal with large numbers

A few shaders were set to use mediump, even though they dealt with
numbers around 2M in magnitude. mediump floats are only guaranteed to
provide range up to 16K, and so the shaders were only reliable on desktop
GPUs which do not optimize mediump. Change these shaders to use highp.
This should fix compatibility with some mobile GPUs.
Merge pull request #1 from NVIDIA/use-highp-when-needed

Use highp in shadow shaders which deal with large numbers