use multiple threads for scanline-based texture mapping

This patch changes the scanline texture mappers to render textures using multiple threads, where the number of threads is determined
using QThread::idealThreadCount().

Almost all rendering logic has been moved into QRunnable implementations, which are recreated for every frame. Since each QRunnable
instantiates its own ScanlineTextureMapperContext, the contexts are recreated for every frame as well.

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