Support transparent shadow

[zhuxudong]

Please check if the PR fulfills these requirements

RFC: https://yuque.antfin-inc.com/shensi.zxd/ftoum6/msk7bmlizt1zlp1h
Playground: galacean/galacean.github.io#1011
dither algorithm compare:https://bartwronski.com/2016/10/30/dithering-part-three-real-world-2d-quantization-dithering/

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

What is the current behavior? (You can also link to an open issue here)

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Other information:

[codecov]

Codecov Report

Attention: Patch coverage is 12.50000% with 7 lines in your changes are missing coverage. Please review.

Please upload report for BASE (dev/1.3@f465149). Learn more about missing BASE report.

❗ Current head ba15e9b differs from pull request most recent head 5e2e37a

Please upload reports for the commit 5e2e37a to get more accurate results.

Files	Patch %	Lines
packages/core/src/Scene.ts	14.28%	6 Missing ⚠️
...ckages/core/src/shadow/CascadedShadowCasterPass.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             dev/1.3    #2080   +/-   ##
==========================================
  Coverage           ?   66.06%           
==========================================
  Files              ?      460           
  Lines              ?    23032           
  Branches           ?     3376           
==========================================
  Hits               ?    15216           
  Misses             ?     6651           
  Partials           ?     1165           

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