modules/shapes: cache per-pixel atan2 results

computing the angle for every pixel coordinate from the origin is
costly, even with the approximate method added by c690303.

An easy speedup is to only do this once for a given frame
dimensions, and cache those results.  In the form of a 32-bit
float, it's equivalent to caching a full page of pixel data.

This is slightly complicated by needing to be an effectively
global cache and the potential for multiple shapes contexts
rendering concurrently when part of a composition.

I think this particular situation highlights a need for something
equivalent generalized on-stream where modules can register
discoverable caches of costly to compute information, having a
high probability of being useful to others.

In this particular case it was alphazed's use of shapes in two
layers that made it an obvious win, even without any other
modules needing atan2() per-pixel with a centered origin.

With this commit:

Configured settings as flags:
  --seed=0x64adabae '--module=compose,layers=blank\,shapes\\\,type\\\=pinwheel\\\,scale\\\=.9\\\,pinch\\\=.25\\\,pinch_spin\\\=.1\\\,pinches\\\=7\\\,points\\\=19\\\,spin\\\=.01\,shapes\\\,type\\\=pinwheel\\\,scale\\\=.9\\\,pinch\\\=1\\\,pinch_spin\\\=-.25\\\,pinches\\\=8\\\,points\\\=5\\\,spin\\\=0,texture=moire\,centers\=3' '--video=mem,size=1366x768'
  FPS: 73
  FPS: 74
  FPS: 73

Without:

Configured settings as flags:
  --seed=0x64adb857 '--module=compose,layers=blank\,shapes\\\,type\\\=pinwheel\\\,scale\\\=.9\\\,pinch\\\=.25\\\,pinch_spin\\\=.1\\\,pinches\\\=7\\\,points\\\=19\\\,spin\\\=.01\,shapes\\\,type\\\=pinwheel\\\,scale\\\=.9\\\,pinch\\\=1\\\,pinch_spin\\\=-.25\\\,pinches\\\=8\\\,points\\\=5\\\,spin\\\=0,texture=moire\,centers\=3' '--video=mem,size=1366x768'
  FPS: 55
  FPS: 54
  FPS: 54

So it's significant, and in alphazed there's also a transition
from one scene with two full-screen shapes layers into a
checkered scene with shapes as the fill_module.  Further
amplifying the payoff..  infact whenever shapes is used for a
fill_module in checkers, there's n_cpus shapes contexts created
because checkers is threaded.  All of those would be benefitting
from the cache.

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