Add 2x2 shading rate generation and management functionality for VRS testing

[Copilot]

Summary

This PR implements functionality to generate, save, and load a full-screen 2x2 shading rate pattern for Variable Rate Shading (VRS) testing in the Vulkan demo application.

Changes Made

Backend Implementation

• Added generate2x2ShadingRate(): Creates a full-screen 2x2 shading rate pattern by filling the entire shading rate image with VK_FRAGMENT_SHADING_RATE_2X2_BIT_KHR (value 0x5)
• Added save2x2ShadingRate(): Generates and saves the 2x2 pattern to a dedicated file (shading_rate_2x2.dat) with proper metadata validation
• Added load2x2ShadingRate(): Loads the saved 2x2 pattern with compatibility checks for dimensions and format

Native Interface Integration

• Added Save2x2ShadingRate() and Load2x2ShadingRate() NAPI functions in PluginRender class
• Exported new functions through the native interface for JavaScript access
• Follows existing error handling patterns with proper logging

UI Enhancements

• Added "Generate & Save 2x2" button (red color) to create and save the special shading rate pattern
• Added "Load 2x2 Shading Rate" button (blue color) to load the previously saved pattern
• Maintained consistent styling with existing UI elements
• Added proper event handling and console logging

Technical Details

The 2x2 shading rate reduces rendering resolution to 1/4 (2x2 pixel blocks share the same fragment shader execution), making it ideal for performance testing scenarios. The implementation:

• Uses a separate file path to avoid conflicts with regular shading rate saves
• Maintains full compatibility with existing VRS functionality
• Includes comprehensive error handling and logging
• Follows the established code patterns and naming conventions

Testing

The functionality can be tested by:

1. Running the application with VRS enabled
2. Clicking "Generate & Save 2x2" to create the pattern
3. Clicking "Load 2x2 Shading Rate" to apply the saved pattern
4. Observing the visual difference in rendering quality (should show 2x2 block artifacts)

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