Add new setting for the number of preloading worker threads

If you have CPU cores to spare, consider setting 2 or 3. Up to about 3 threads, preloading performance seems to increase in a linear fashion, but with 4 or more threads I/O bottlenecks and synchronization overhead starts to show.

apps/openmw/mwrender/renderingmanager.cpp

@@ -158,7 +158,7 @@ namespace MWRender
         : mViewer(viewer)
         , mRootNode(rootNode)
         , mResourceSystem(resourceSystem)
-        , mWorkQueue(new SceneUtil::WorkQueue)
+        , mWorkQueue(new SceneUtil::WorkQueue(Settings::Manager::getInt("preload num threads", "Cells")))
         , mUnrefQueue(new SceneUtil::UnrefQueue)
         , mFogDepth(0.f)
         , mUnderwaterColor(fallback->getFallbackColour("Water_UnderwaterColor"))

files/settings-default.cfg

@@ -42,6 +42,9 @@ exterior cell load distance = 1
 # Preload cells in a background thread. All settings starting with 'preload' have no effect unless this is enabled.
 preload enabled = true
 
+# The number of threads to be used for preloading operations.
+preload num threads = 1
+
 # Preload adjacent cells when moving close to an exterior cell border.
 preload exterior grid = true