perf(module:qr-code): improved background drawing efficiency

[HyperLife1119]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.57%. Comparing base (65cbb4e) to head (3d76b08).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8543      +/-   ##
==========================================
- Coverage   91.57%   91.57%   -0.01%     
==========================================
  Files         532      532              
  Lines       18334    18332       -2     
  Branches     2801     2801              
==========================================
- Hits        16790    16788       -2     
  Misses       1226     1226              
  Partials      318      318              

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[OriginRing]

LGTM