perf: release asset buffers once assets are loaded

[hannojg]

In the clients app I can tell that the buffers are seldomly cleaned up. It easily lets the app allocate multiple GBs of memory before considering cleaning those up.

Actually we need a mechanism to cleanup everything, not just the buffers. Use case is for instance:

• Opening a new screen with Filament + Engine
• Leaving that screen (screen is destroyed)
• Engine + Filament view not getting cleaned up immediately and might be dangling for super long

That seems to involve some more elaborate mechanisms such as a dispose API that can be called from JS or using native state + adding memory pressure to the NativeState object.