fixed scaleFromCenter and aspectRatio bug

[beasteers]

the scaleFromCenter and aspectRatio flags weren't working as I expected. I thought they would be flags that changed the default behavior of the transformation, but instead it was just the initial value from the mousedown event.

To fix it, I refactored a bit. Instead of manually specifying this

scale(scaleType, {
    startX: event.pageX, 
    startY: event.pageY, 
    startFromCenter: event.altKey,
    aspectRatio: event.shiftKey,
    ...
})

you can just do this:

scale(scaleType, {
    event,
    ...
})

That allows us to utilize startFromCenter and aspectRatio for specifying their default behavior. If we set either to true, alt/shift will still toggle, just in the other direction.
For the future, another capability that might be nice would be to allow startFromCenter and aspectRatio to be locked. Currently there is enableStartFromCenter=false which would disable scale from center, but it might be nice to be able to disable scale from corner and only allow scaling from the center. An easy thought would be to use something along the lines of enableStartFromCenter=[false|null|true] for specifying [scale from corner, toggleable, scale from center], though naming that argument might be a challenge.

[codecov-io]

Codecov Report

Merging #1 into master will increase coverage by 0.64%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master     #1      +/-   ##
========================================
+ Coverage   99.35%   100%   +0.64%     
========================================
  Files           5      5              
  Lines         156    158       +2     
  Branches       30     28       -2     
========================================
+ Hits          155    158       +3     
+ Misses          1      0       -1

Impacted Files	Coverage Δ
src/scale.js	100% <100%> (+2.22%)	⬆️

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