Make bloom post process check for selected feature

Right now you can apply certain post process effects to just one feature. The bloom shader doesn't do any check to allow you to apply it to just to the selected feature as you would expect.

This is potentially pretty easy, you just need to take the check from another post process like the black and white one: https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Shaders/PostProcessStages/BlackAndWhite.glsl

and apply it in the bloom shader: https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Shaders/PostProcessStages/BloomComposite.glsl

The [forum thread](https://groups.google.com/d/msg/cesium-dev/GAr3d-TgQq4/ux-cxNxlDwAJ) here has an example you could use for testing. Note that you'd need to test the selected feature on a model, since entities aren't yet supported there (see https://github.com/AnalyticalGraphicsInc/cesium/issues/6705).

Compare Source/Shaders/PostProcessStages/BlackAndWhite.glsl with Source/Shaders/PostProcessStages/BloomComposite.glsl and trace how the selected-feature check is applied. Use the forum thread's example with a model, then verify that bloom affects only the selected feature and leaves other model parts unchanged.

Written by the indexing model from the issue text.