plants compute shader

The retarget's foot contact analysis: one invocation per foot over the per-frame foot and body positions the transfer kernel wrote, the target's and the source's. The source states where the feet are down: its plants are the frames a foot's contact point (the lower of ankle and toe) is near the floor and still against the ground. It measures the clip's travel speed on the target over the frames the source has a foot down (the body's forward speed against that foot's), so an in-place clip and one with root motion both answer, and the worst slide of the target's contact point within a plant. Run again after the ik kernel, it measures the solved slide and each leg's joint deviation. modules/retarget reads the result back beside the retargeted clip.