Instead of the loadscreen eager-loading ALL level chunks to Go Images, only
load the chunks within the "LoadingViewport" - which is the on-screen
Viewport plus a margin of chunks off the screen edges.
During gameplay, every few ticks, reevaluate which chunks are inside or
outside the LoadingViewport; for chunks outside, free their SDL2 textures
and free their cached bitmaps to keep overall memory usage down. The
AzulianTag-Forest level now stays under 200 Textures at any given time
and the loadscreen goes faster as it doesn't have to load every chunk's
images up front.
The LoadUnloadChunk feature can be turned on/off with feature flags. If
disabled the old behavior is restored: loadscreen loads all images and
the LoadUnloadChunks function is not run.
Other changes:
* loadscreen: do not free textures in the Hide() function as this runs on
a different goroutine and may break. The 4 wallpaper textures are OK
to keep in memory anyway, the loadscreen is reused often!
* Free more leaked textures: on the Inventory frame and when an actor
calls Self.Destroy()
* Stop leaking goroutines in the PubSub feature of the doodad script
engine; scripting.Supervisor.Teardown() sends a stop signal to all
scripts to clean up neatly. Canvas.Destroy() tears down its scripting
supervisor automatically.
