modules/flow: implement threaded rendering

This exploits the just added multipass rendering support.

In the first pass, the flow-field is sampled and applied to the
elements, with every thread operating on its own subset of the
elements list.  Since the flow-field sampling is all read-only,
it's perfectly safe too do in parallel.  Nothing is drawn in the
first pass, it's only the elements updating according to the
flow-field which is performed.

In the second pass, the elements are rendered in parallel using
the slice_per_cpu fragmenter.  Since the elements are kept on a
simple array, with no spatial indexing, every thread must visit
every element.

Since the fragmenter used divides the frame into horizontal
slices, every thread needing to reject elements not overlapping
its region can take some shortcuts in easily identifying elements
entirely outside its region.  But the whole 3d->2d projection
step must still be performed for every element's current position
and +n_iters final position for the frame, which does have a
divide unfortunately.

Nonetheless, this change improves frame rates substantially on my
2c/4t i7 X230 as benchmarked w/--video=mem,1366x768:

  --seed=0x64fa9508 '--module=rtv,channels=flow,duration=3,context_duration=3,caption_duration=0,log_channels=on,snow_duration=0,snow_module=none' '--video=mem,size=1366x768'

rtv channel settings: 'flow,size=4,count=40000,speed=.8'
FPS: 261
FPS: 265
rtv channel settings: 'flow,size=4,count=1000,speed=.9'
FPS: 1153
FPS: 3204
FPS: 2934
rtv channel settings: 'flow,size=8,count=5000,speed=.9'
FPS: 2923
FPS: 1634
FPS: 1592
rtv channel settings: 'flow,size=2,count=50000,speed=.4'
FPS: 1006
FPS: 219
FPS: 268
rtv channel settings: 'flow,size=16,count=30000,speed=.8'
FPS: 304
FPS: 350
FPS: 343
rtv channel settings: 'flow,size=16,count=30000,speed=.02'
FPS: 379
FPS: 503
FPS: 472
rtv channel settings: 'flow,size=8,count=1000,speed=.16'
FPS: 1393
FPS: 3822
FPS: 3876

---

Prior to this commit:
  --seed=0x64fa9508 '--module=rtv,channels=flow,duration=3,context_duration=3,caption_duration=0,log_channels=on,snow_duration=0,snow_module=none' '--video=mem,size=1366x768'

rtv channel settings: 'flow,size=4,count=40000,speed=.8'
FPS: 53
FPS: 53
rtv channel settings: 'flow,size=4,count=1000,speed=.9'
FPS: 426
FPS: 1366
FPS: 1335
rtv channel settings: 'flow,size=8,count=5000,speed=.9'
FPS: 1097
FPS: 368
FPS: 367
rtv channel settings: 'flow,size=2,count=50000,speed=.4'
FPS: 279
FPS: 73
FPS: 74
rtv channel settings: 'flow,size=16,count=30000,speed=.8'
FPS: 71
FPS: 71
FPS: 70
rtv channel settings: 'flow,size=16,count=30000,speed=.02'
FPS: 136
FPS: 305
FPS: 305
rtv channel settings: 'flow,size=8,count=1000,speed=.16'
FPS: 972
FPS: 2593
FPS: 2634

0 files changed, 0 insertions, 0 deletions