GPU Shaders and Pipelines aren't cleaned up when the relevant assets are removed

This means that as shaders and pipelines are loaded/unloaded, we slowly accumulate allocated gpu resources.

Now that we have live-shader reloading merged (#937), this is particularly important.

Start by tracing the shader and pipeline asset load and unload paths, then review the live-shader reloading work referenced in #937. Done means removing the relevant assets also releases their allocated GPU resources, including across repeated shader and pipeline reloads.

Written by the indexing model from the issue text.