Make the perspective transformations use eigen2 instead of old cut-and-pasted code.
Removes 450 LOC.

Use a 9x9 fixed-size matrix, instead of 10x10 dynamic-size (I don't understand why it was 10x10, perhaps I'm missing something).

Rework the API a bit, so functions which returned a pointer to dynamically allocated memory now return a fixed-size Eigen matrix. This fixes a few memleaks as these pointers were not always deleted.

Warning! The unit-test is empty, and I since I don't know how that feature is supposed to behave, I can't be sure that I didn't break it.
Please test!

Eigen2 now mandatory for building Krita. ${SARDONIC_LAUGHTER}