QtBiomeVisualiser: Switched caching to entire regions.

This speeds up the rendering preparation for small zooms.
QtBiomeVisualiser: Attempt at fixing Linux compilation.
Merged remote-tracking branch 'origin/master'.
QtBiomeVisualiser: Fixed random regions not rendered.
QtBiomeVisualiser: Reduced memory usage.
Merge pull request #1574 from mc-server/QtBiomeVisualiserRegions

QtBiomeVisualiser: Switched caching to entire regions.