Round data to nearest integer before converting to 8bit

[dionhaefner]

Fixes #249

[codecov]

Codecov Report

Merging #250 (7daa644) into main (04f1019) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #250   +/-   ##
=======================================
  Coverage   98.13%   98.13%           
=======================================
  Files          45       45           
  Lines        2253     2254    +1     
  Branches      329      329           
=======================================
+ Hits         2211     2212    +1     
  Misses         24       24           
  Partials       18       18           

Impacted Files	Coverage Δ
terracotta/cache.py	100.00% <100.00%> (ø)
terracotta/drivers/mysql.py	100.00% <100.00%> (ø)
terracotta/image.py	100.00% <100.00%> (ø)

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