This commit is meant to improve the response of the ESC key for stopping Freestyle rendering
throughout the rendering process. The rendering with Freestyle consists of several steps
including: (1) mesh data loading, (2) winged edge construction, (3) silhouette edge detection,
(4) view map construction, and (5) stroke drawing. All these steps have been extended to
frequently check if the ESC key is pressed, so that users can abort time-consuming rendering
at any point of time.
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