Improve normalized stack bar performance

[panthony]

I had a huge performance regression for normalized stack bar when I have a thousand bar and ~10 series.

It makes the browser hang for a while, stuck in 'filterTargetsToShow' mainly.

When I investigated I found that:

• the calls to 'api.data.shown' seems unnecessary.
• we normalized values for a lot of unnecessary targets. Like a LOT, coupled with the fact that 'filterTargetsToShow' were copying the targets it had a huge hit

[codecov-io]

Codecov Report

Merging #2699 into master will decrease coverage by 0.04%.
The diff coverage is 90.32%.

@@            Coverage Diff             @@
##           master    #2699      +/-   ##
==========================================
- Coverage   82.68%   82.64%   -0.05%     
==========================================
  Files          59       59              
  Lines        4718     4724       +6     
==========================================
+ Hits         3901     3904       +3     
- Misses        817      820       +3

Impacted Files	Coverage Δ
src/shape.js	95.52% <100%> (+0.13%)	⬆️
src/data.js	88.34% <83.33%> (-0.79%)	⬇️

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