Fine grained progress + cancellation

[krupkat]

Fine grained progress bar when stitching, what was previously counted as a single step is now (8 + n_images):

• finding features
• matching features
• estimating homography
• bundle adjustments
• preparing seam optimization
• seam optimization
• compose (n_images steps)
• blend
• finalize

There are also cancel checkpoints between all the steps, so that the computation can be quickly abandoned. This is useful when launching new stitching tasks e.g. when manually editing a panorama by ctrl clicking, see the video for an example:

xpano_cancel.mp4

images credit: Steven from https://monarophotography.au/

This feature required two big changes:

• use a fork of the cv::Stitcher class to be able to add the progress monitoring and cancel checkpoints
• refactor StitcherPipeline to manage a queue of Tasks, each of them with a separate ProgressMonitor and a cancellation flag

Using the forked Stitcher class will possibly allow modification for a 16-bit pipeline (#74)

Fixes #73