Allow for custom imagery adjustments

Provide a hook into the shader for custom adjustments to imagery when rendering the surface of the globe.

From the forum: https://groups.google.com/forum/#!topic/cesium-dev/ZCXAoriJogw

> Imagery layer already provides some built in adjustments, like brightness/contrast/hue/saturation, which I believe are performed by shaders in the rendering pipeline. What is the best way to add my own custom adjustments to an imagery layer. For example, I would like to dynamically adjust the brightness of pixels per RGB channel (brighten blue, dim red, etc). Is there any way to add my own shader for a specific imagery layer (and pass parameters to it)? Or perhaps, is there another way to do this without shaders?

Start by reviewing the imagery-layer rendering path and the shader stage used for the globe surface; no specific files or tests are named in the issue. Use the linked forum discussion to understand the requested per-layer RGB adjustments and parameter passing, then define an API and verification approach for custom imagery shaders.

Written by the indexing model from the issue text.