The worker currently works by receiving GENERATE_BUFFERS messages and
will send back the same kind of message, with the generated buffers
attached. All properties of the original message are kept, so that
when a GENERATE_BUFFERS message comes back to the main thread it
is possible to know what and how the buffers where generated.
This is typically used for the `projectionTransform` matrix, and
will also be necessary when working with tiles.
The worker receives a transferable array of instructions
and sends back two transferable arrays (vertex and index buffer).
The projection transform is also sent so that when the main thread
receives the buffers from the worker it also knows which projection to
apply when rendering the geometries.
