Add transparency mask support for /compute

[dionhaefner]

#221

[codecov]

Codecov Report

Merging #222 (930c63f) into main (8bdd1f1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   98.37%   98.37%           
=======================================
  Files          45       45           
  Lines        2216     2217    +1     
  Branches      275      275           
=======================================
+ Hits         2180     2181    +1     
  Misses         20       20           
  Partials       16       16           

Impacted Files	Coverage Δ
terracotta/expressions.py	100.00% <100.00%> (ø)

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