Cleanup (and make slightly faster as a side effect) the way VSE effects
do multi-threading. Previously (some of them) were using
IMB_processor_apply_threaded with C-like machinery (which internally
uses a task pool), switch that over to a helper apply_effect_op
(which internally uses a parallel for). Based on profiling, parallel
for is slightly more efficient (task pool takes a bit until all the
tasks are "pushed" into the pool). Note however that some VSE effects
were already doing parallel for internally; these are not affected.
VSE scene at 4K resolution, with four 4K resolution PNG images blended
over each other, time it takes to do render_strip_stack:
- Ryzen 5950X (Win/VS2022): 38.9ms -> 34.7ms
- Mac M4 Max: 21.9ms -> 19.8ms
Now that all VSE effects are internally threaded via parallel for,
there's no need for the init_execution and execute_slice machinery,
so remove all that.
You might also notice that half of "over drop" effect code is gone.
It was accidentally not doing anything whatsoever for the last 18 years
(since 2.42), and currently observed behavior matches documentation
and "internet knowledge", so let's accept it as correct.
Pull Request: https://projects.blender.org/blender/blender/pulls/132380
c26db69f58