Simplify star recomposition

This MR simplifies the star recomposition tool by removing the crossfader and final stretch tools and replace by combining the two images in LCh space with a L combination that cannot exceed 1.f (this combination is automatic and no user interaction is required).

I investigated using OpenCV but there was no benefit as there are no optimised element-wise trig functions that apply to an entire Mat.

So instead, optimisation is by means of using the trig functions in sleef.h, and OpenMP. I'm still tweaking the optimisation to simplify a couple of calculations. I also plan to test using LAB space instead of LCh space, which would further reduce the amount of expensive functions used.

The current version of sleef is much newer and more optimised than the bundled core/sleef.h and has both scalar and vectorised math functions (and the scalar ones are written so as to auto-vectorise easily) optimised for a lot of architectures, so updating to the most recent sleef as a subproject or additional dependency is an avenue for further optimisation throughout Siril (and also provides NEON support for acceleration of maths on ARM devices). A possible further development for 1.4?