cannot cancel tool operations
Submitted by Jakub Steiner
When applying a filter, the user has the chance to cancel the process by clicking on the 'Cancel' button on the status bar of the image window.
However when a tool operation is being processed, it cannot be aborted this way. There are some actions that take quite a long time depending on the image size and should be abortable (gradients with adaptive supersampling, transofrmation of large images /rotation, scaling, perspective.../).
Version: git master