chore: improve ImageAssert.AreSimilarAsync perf

[Xiaoy312]

GitHub Issue: n/a

PR Type: 🔄 Refactoring (no functional changes, no api changes)

What is the current behavior? 🤔

What is the new behavior? 🚀

dont access the same pixel multiple times during ImageAssert.AreSimilarAsync

[Copilot]

Pull request overview

This PR optimizes the ImageAssert.AreSimilarAsync method by caching pixel values before performing calculations, reducing redundant indexer calls during image comparison. The RawBitmap indexer performs scaling calculations, array offset computation, and Color object construction, making repeated accesses expensive. By caching each pixel value once instead of accessing it three times, this change improves performance in a hot loop that processes every pixel in the image.

Changes:

• Cache expected[x, y] and actual[x, y] pixel values before calculations
• Use cached pixel values for RGB and alpha component calculations, reducing indexer calls from 6 to 2 per pixel

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