cssom: support parsing background-image and render as a grad effect

[yorkie]

No description provided.

[Copilot]

Pull Request Overview

This PR introduces CSS image parsing support for background-image properties with gradient rendering capabilities. The implementation adds proper CSS tokenization for image values and provides a gradient effect placeholder in the renderer.

• Implements CSS image parser with support for url(), src(), image-set() and gradient functions
• Adds comprehensive test coverage for CSS value tokenization and image parsing
• Integrates background-image support into the computed style system with Skia gradient rendering

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/client/css_image_parser_tests.cpp	Comprehensive test suite for CSS tokenizer and image parser functionality
src/client/cssom/values/specified/image.hpp	Enhanced image value class with parsing and serialization capabilities
src/client/cssom/values/specified/image.cpp	Implementation of image parsing, CSS serialization and computed value conversion
src/client/cssom/values/generics/image.hpp	Extended generic gradient classes with radial gradient support
src/client/cssom/values/computed/image.hpp	Computed image value definitions
src/client/cssom/parsers/css_value_tokenizer.hpp	CSS value tokenizer for parsing image syntax
src/client/cssom/parsers/css_value_tokenizer.cpp	Complete CSS tokenizer implementation
src/client/cssom/parsers/css_image_parser.hpp	CSS image parser interface
src/client/cssom/parsers/css_image_parser.cpp	Full CSS image parsing implementation
src/client/cssom/computed_style.hpp	Added background-image property support
src/client/builtin_scene/web_content_renderer.cpp	Integrated gradient rendering with Skia
cmake/TransmuteClient.cmake	Restructured build system to support library linking

Comments suppressed due to low confidence (1)

tests/client/css_image_parser_tests.cpp:201

• These placeholder tests don't verify the actual parsing behavior. Replace with meaningful assertions that check if the parsed image has the correct type and properties.

    REQUIRE(true); // Placeholder - actual implementation would check image.isUrl() etc.