feat: improve viewport scrolling

[ktsn]

fix #20

This is an initial implementation to deal with Photoshop-like viewport scrolling.

If the viewport is enough smaller than the window size, it does not allow scroll, otherwise the wrapper element of viewport will have much larger size than window so that we can scroll it in larger area.

Maybe I will refactor the code since the code is a bit messy.

[ktsn]

[codecov]

Codecov Report

Merging #24 into master will increase coverage by 0.92%.
The diff coverage is 96%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage    64.2%   65.13%   +0.92%     
==========================================
  Files          44       44              
  Lines        1369     1394      +25     
  Branches      315      317       +2     
==========================================
+ Hits          879      908      +29     
+ Misses        475      471       -4     
  Partials       15       15

Impacted Files	Coverage Δ
src/view/components/Renderer.vue	96.42% <96%> (+96.42%)	⬆️
src/view/components/Viewport.vue	100% <0%> (+100%)	⬆️

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