use fast path in blendSegment to bump up FPS

[DedeHai]

currently blendSegment is "the bottleneck" in the rendering pipeline. I ran a breakdown of times (in µs):

• total=8958
• effect rendering: 6156
• show: 2677 = blend 1359 + pixelstobus 1318

this is for a single segment. blending is slow for a "just copy the segment buffer"

with this PR it becomes:

• total=8091
• effect rendering: 6536 (fluctuates)
• show: 14258 = blend 105 + pixelstobus 1320

Improvements in FPS:

• 2 overlapping segments: 76.6FPS -> 84.5FPS
• single segment: 103FPS -> 110FPS

i.e. up to 10% faster.

test run on C3, 2 outputs 256 pixels each, 32x16 matrix using PS-Fire

Summary by CodeRabbit

• Refactor
  • Optimized LED animation rendering performance through enhanced pixel blending operations and memory management, particularly improving fade transition efficiency when grouping and mirroring features are not in use.
  • Streamlined pixel clearing and processing operations to reduce computational overhead and improve overall animation rendering speed while maintaining backward compatibility with all existing effects.

[coderabbitai]

Walkthrough

This PR optimizes the segment blending pipeline by introducing a fast-path in blendSegment() for fade transitions without grouping, mirroring, or CCT pixels, and replaces the per-element pixel-clearing loop with memset() in show().

Changes

Cohort / File(s)	Summary
Segment Blending Optimization wled00/FX_fcn.cpp	Introduced fast-path optimization in blendSegment() bypassing slow blending logic when conditions permit (no old segment, fade transition, no grouping/mirroring/CCT); includes special handling for Reverse/ReverseY and 2D transpose; refactored progress/opacity computation; removed redundant variable declaration in slow path.
Pixel Clearing Efficiency wled00/FX_fcn.cpp	Replaced per-element loop clearing _pixels[i] = BLACK with single memset() call in show().

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• apply inverse gamma to segment brightness for better color preservation #5343 — Modifies opacity/brightness computation in WS2812FX::blendSegment() during blending, overlapping logic area with this PR's fast-path changes.
• Stencil blendingmode and some speed optimisations #4889 — Modifies WS2812FX::blendSegment() and transition-style handling, including blend-mode selection and TRANSITION\_* usage patterns.
• Bugfix for brightness factor upon save: fixes #4824 #4827 — Modifies wled00/FX_fcn.cpp pixel-blending and clearing logic, directly overlapping with this PR's optimization targets.

Suggested reviewers

• willmmiles
• softhack007
• blazoncek

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: introducing a fast path in blendSegment to improve FPS performance, which is the primary objective of the pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[DedeHai]

just FYI: ported this improvement over to my WLEDbus driver branch, I do get ~100FPS for the 2 overlapping segments which come in at 84.5FPS using NPB.