Flush texture caches

[jhodgson]

We need a way to flush the texture caches in the render delegate. We'll need a way to interrupt the render, flush the caches and restart. Options should include flushing textures, quad lights, skydomes, all caches, and caches for specific textures.

[autodesk-oss-arnold-bot[bot]]

Issue synced internally to ARNOLD-13262