Explicitly compute overview levels

[dionhaefner]

Based on cogeotiff/rio-tiler#132

Overall some performance improvements, but we will have to test more to see whether it is worth it.

[codecov]

Codecov Report

Merging #158 into master will decrease coverage by 0.21%.
The diff coverage is 72.22%.

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
- Coverage   98.25%   98.03%   -0.22%     
==========================================
  Files          43       43              
  Lines        2122     2140      +18     
  Branches      260      266       +6     
==========================================
+ Hits         2085     2098      +13     
- Misses         20       22       +2     
- Partials       17       20       +3

Impacted Files	Coverage Δ
terracotta/drivers/raster_base.py	93.97% <72.22%> (-1.49%)	⬇️

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[dionhaefner]

No benefit with the new default transform.