Fix the redrawing of threaded GL canvases
QtThreadedCanvas used to ignore all repaint events send to it by Qt in order to avoid problems with threading. As a result, however, it would become blank when docked/undocked, and display garbage in those of its regions that some other widgets have moved over. To fix this, the API of TGT's Painter class had to be slightly modified and now mirrors that of Canvas with regard to painting-related operations. The paint method has been made protected; it's supposed to implement the painting logic and shouldn't be called directly. A public repaint method has been introduced to let canvases notify their associated painters that they need to be redrawn. The default implementation simply calls paint() immediately; threaded painters, however, override it to schedule render jobs that run in separate threads. All existing threaded painters have been updated to reduce the visibility of their paint methods and provide public repaint functions. Consequently, QtThreadedCanvas can now properly handle repaint events by delegating to its associated painter's repaint method.
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