Add compute render pass

[pjcozzi]

Commands for this pass will execute before executing any commands to render the scene. This will eliminate the direct calls to Context.draw in the imagery code.

[pjcozzi]

We should be able to use the post-processing framework to drive this (at the start of the frame) since our compute passes are viweport-aligned quads.

[pjcozzi]

@lilleyse for this, review the notes on the compute pass in Rendering a Frame, and our two current use cases:

• Sun - Line 218
• ImageryLayer - Line 962

The general idea is only the scene should execute commands - primitives should not. If a primitive has a GPGPU-like task (which results in updating a texture) then they should create a new ComputeCommand for it, return it from update using a new Pass.Compute pass, and then receive a callback/promise when ready. The callback/promise will be allowed to return new commands to be executed that frame...perhaps even compute commands...I need to think about that.