feat: backdrop blurring and blend modes

[Tropix126]

This PR adds two new properties:

• backdrop_blur, which applies a blur effect to the contents behind a rect's background.
• blend_mode, which allows you to change the interaction between an element's colors and the contents behind it.

Todo

• Figure out why these two properties don't like to work together (blur doesn't apply on blend modes). This is either an implementation detail of skia leaking through (this same bug occurs on chromium, actually) or it's me misunderstanding how layer saving works.
• Verify that backdrop_blur works with opacity.
• examples
• docs

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 96 lines in your changes missing coverage. Please review.

Project coverage is 75.71%. Comparing base (3078a28) to head (fe3a064).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/elements/rect.rs	0.00%	45 Missing ⚠️
crates/state/src/values/blend_mode.rs	0.00%	33 Missing ⚠️
crates/core/src/render/pipeline.rs	0.00%	10 Missing ⚠️
crates/state/src/style.rs	0.00%	6 Missing ⚠️
crates/native-core/src/attributes.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #924      +/-   ##
==========================================
- Coverage   75.99%   75.71%   -0.28%     
==========================================
  Files         207      208       +1     
  Lines       23775    23861      +86     
==========================================
  Hits        18067    18067              
- Misses       5708     5794      +86     

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