Reuse asset optimization cache between release and debug builds

[ealmloff]

Feature Request

We skip re-optimizing assets when we see the file in the build cache. The asset name has the hash of the input contents and asset options so if we see the same name, the optimized version is always the same. However since we output assets to a different place in debug vs release builds, we end up re-optimizing assets when switching between the two builds

Implement Suggestion

Use a shared location for the asset optimization cache and copy assets from there into the final build