Perf: Optimize Screenshot diff check

[jeremiahseun]

💡 Motivation and Context

This PR optimises the Session Replay screenshot capturing logic to reduce CPU usage and improve performance.
Previously, we were encoding the screen snapshot to PNG format before checking if the screen had actually changed. PNG encoding is a computationally expensive operation.
The Fix:
I switched the "diff check" logic to use ui.ImageByteFormat.rawRgba instead of PNG. The Raw RGBA format is significantly faster to generate as it requires no compression. We now only proceed to the expensive PNG encoding step after we have confirmed that the new frame is different from the previous one.
This change avoids unnecessary PNG encoding cycles when the UI is static, leading to smoother app performance during recording.

💚 How did you test it?

I added two new test files to verify both the performance gain and the correctness of the implementation:

1. test/benchmark_screenshot_performance_test.dart: A benchmark test that compares the speed of "PNG Encoding + Compare" vs "Raw RGBA Encoding + Compare".
   • Result: Confirmed a significant speedup factor (e.g., ~15-20x faster) for the diffing step.
2. test/validation_screenshot_diff_test.dart: A validation test that ensures rawRgba bytes are identical for identical images and different for distinct images.
   • Result: Verified that the optimisation does not introduce false positives or negatives in change detection.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

Screenshots

[marandaneto]

@jeremiahseun mind adding a changelog entry under next? https://github.com/PostHog/posthog-flutter/blob/main/CHANGELOG.md#next

[marandaneto]

thanks @jeremiahseun

[jeremiahseun]

thanks @jeremiahseun

Sure @marandaneto
Should I leave those test files or I should remove them?
test/benchmark_screenshot_performance_test.dart
test/validation_screenshot_diff_test.dart

[marandaneto]

thanks @jeremiahseun

Sure @marandaneto Should I leave those test files or I should remove them? test/benchmark_screenshot_performance_test.dart test/validation_screenshot_diff_test.dart

i think you can remove both

[jeremiahseun]

@marandaneto I have updated the CHANGELOG and removed the test files.