Refactor out HeadlessFrontend and HeadlessBackend gl independent code to gfx.
Define gl::HeadlessBackend as subclass, instantiated by gfx::HeadlessBackend static factory method.
GL dependent tests are still using gl::HeadlessBackend directly (not through gfx).
Add new APIs for invalidating tiles, effectively forcing Mapbox
GL Native to check with the servers if the tiles are valid before
using them. This is more efficient then deleting tiles, because
in case of valid tiles, they won't get downloaded.
Fixes#4376.
- 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`.
