improved mountain mask

[MM1nd]

In absolute timing this takes about 10s off the clock for a 1024*512 map. Relative timings are dominated by anti-aliasing, but I thought I'd do this while waiting for #251 to be resolved.

Code is also a bit cleaner, once again.

Not much more to be said about this one.

[coveralls]

Coverage increased (+0.1%) to 85.011% when pulling 99ed82f on MM1nd:mountains_mask into 20d93d9 on Mindwerks:master.

[codecov-io]

Codecov Report

Merging #252 into master will increase coverage by 0.86%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #252      +/-   ##
=========================================
+ Coverage   79.64%   80.5%   +0.86%     
=========================================
  Files          28      28              
  Lines        3620    3617       -3     
  Branches      721     715       -6     
=========================================
+ Hits         2883    2912      +29     
+ Misses        547     511      -36     
- Partials      190     194       +4

Impacted Files	Coverage Δ
worldengine/common.py	85.05% <100%> (+0.17%)	⬆️
worldengine/drawing_functions.py	80.8% <100%> (ø)	⬆️
worldengine/model/world.py	75% <100%> (+0.66%)	⬆️
worldengine/simulations/erosion.py	69.96% <0%> (+10.67%)	⬆️

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[coveralls]

Coverage increased (+1.0%) to 85.872% when pulling 9d7d214 on MM1nd:mountains_mask into 20d93d9 on Mindwerks:master.

[psi29a]

Nice work! Those nested loops are gone which is a big win for simplicity. Thank you again. :)