Implemented thread pool (with dummy workers).
Fixed RenderWorker crashing without the progress bool shared pointer.
Using dispatcher in manager.cpp.
Implemented thread workers, added extra work to render other composite
tiles.
Added average speed to progress bar (only temporary).
Fixed new multithreading to properly work with later added work.
(later added work = remaining composite tiles)
Fixed small logic multithreading problem.
extra queue has a higher priority than normal work queue
Removed unnecessary work/result lists.
Added some messages, changed some variable names/orders.
Renamed render.h/cpp to tilerenderer.h/cpp.
Renamed render/ dir to renderer/
Renamed RenderWorker to TileRenderWorker, renamed header/source as well.
Renamed render namespace to renderer.
Renamed cache.h/cpp to worldcache.h/cpp.
Updated GPL headers.
Ordered some includes.
Moved RenderWork* to TileRenderWorker.
Removed old render manager code.
TileRenderWorker uses RenderWork/RenderContext,
renamed RenderWorkContext to RenderContext.
Forgot to commit dispatcher.h last time.