Binary shader support has been removed in commit c2f974f2a5 ([core]
Remove binary shader support). This left-over parameter is not used
anywhere anymore.
- Do not carry it over everywhere as parameter, it is a shared
instance anyway and the lifecycle is pretty much the app lifecycle
from the moment we instantiate a map.
- Rename to BackgroundScheduler because it is a Scheduler that will
do tasks in the background, we don't make assumptions if it is a
thread pool or a single thread.
- Most importantly, remove the dependency from `core` on `platform`.
Render threads won't have a scheduler, so we create one that is
shared by all the maps rendering at this thread. The bad side
effect of this is that we need to wake up the render thread
to process events.
Mapbox GL should get rid of processing events on the render
thread. This solution is a workaround.
