Keyframe Selection: Add multi-threading and improve overall performances

[cbentejac]

Description

This PR speeds up the selection of keyframes on two different accounts:

• During the computation of the sharpness scores, the sliding window used to be moved pixel by pixel, which resulted in very accurate sharpness scores but also expensive computations. As the sharpness score can afford losing a bit of accuracy, the sliding window is now moved across the image by steps of windowSize / 4, with windowSize the size of the sliding window.
• For the computation of all the scores, blocks of frames whose size depends on the number of available threads are computed and then processed in dedicated threads. Blocks are overlapping, since the optical flow requires the current frame N as well as the previous frame N-1.

It relates to alicevision/Meshroom#2161 and should only be merged once #1510 has been integrated into develop.

Features list

• Use a larger step for the sliding window of the sharpness score computation, effectively reducing the computation time for the sharpness score;
• Add multi-threading for the computation of all scores.

Implementation remarks

The parallelization system using chunks that is used in Meshroom cannot be used to speed the keyframe selection up, because only the step of the score computation can be parallelized: once all the scores have been computed, they need to be put back all together sequentially for the keyframes to actually be selected and this process cannot be split.