Optimize Conway with a Summed Area Table

[chrisib]

Simple performance optimization for Conway to use a Summed Area Table instead of iterating through the neighbourhoods of changed pixels. This should keep the framerate more stable.

Replaces #435, though the multi-threaded approach may be reintroduced later.

[chrisib]

Debugging a very strange problem that seems to occur after a few hours of continuous use:

45061: 'bool' object has no attribute 'field_sum'
Traceback (most recent call last):
  File "/lib/bootloader.py", line 186, in main
  File "/lib/contrib/conway.py", line 348, in main
  File "/lib/contrib/conway.py", line 271, in tick
  File "/lib/contrib/conway.py", line 219, in sum_cells
AttributeError: 'bool' object has no attribute 'field_sum'

The stack trace calls are

conway.py", line 348, in main: self.tick()
conway.py", line 271, in tick: num_neighbours = self.sum_cells(top, left, bottom, right)
conway.py", line 219, in sum_cells: a = self.field_sum[start_row - 1][start_col - 1]

It's consistently that line that breaks, but I haven't succeeded in any debugging inspections yet. Launching from the intepreter:

>>> from contrib.conway import Conway
>>> c = Conway()
>>> c.main()
...
AttributeError: 'bool' object has no attribute 'field_sum'

re-running the offending function:

>>> c.tick()

works fine. So it seems to be some sort of runtime memory issue? Maybe there's a pointer that's going squirrely? Something is going wrong, but I haven't figured out exactly what or why yet.

[chrisib]

Was using uPython 1.24.1, so let's see if it goes away after upgrading to 1.25.0...

[chrisib]

Ran it overnight with 1.25.0 and the issue didn't reoccur. Frustrating that I can't figure out what happened, but it seems to be fixed.