Feature Request : looping custom effects

[jules-timmerman]

I really enjoy this fork, but I haven’t found a way to have effects that vary as time goes on.
I believe we could extend the behavior of --effect-custom by adding a switch --effect-custom-loop.
The lib would have a function with the signature looking like struct timespec swaylock_effect(uint32_t *data, int width, int height, int scale) where the returned value would be the expiry value in a struct loop_timer and we would re-execute this function again after expiry run out
I am ok with implementing this but I would love some feedback first.