[impeller] reduce reliance on Reactor

[gaaclarke]

impeller::ReactorGLES was designed as a way to simplify threading and opengles. It does this by providing a collection of handles to opengles objects and storing operations that are flushed on proper opengles threads.

However, for many objects in impeller we can statically reason about their safety. So the added hash table / mutexes to store that threadsafe information are wasted. For example: TextureGLES are objects created and deleted on the raster thread.

Potential objects that can be transitioned

• Textures
• Pipelines
• DeviceBuffer