[Performance] Render layout masks within polygon bounds

[scyyh11]

Layout visualization currently allocates a full-page mask for every polygon, stacks all masks, and scans each full page to locate covered pixels. Documents with many layout regions spend substantial CPU time and memory on this work.

This change rasterizes each polygon into its clipped bounding rectangle and blends that region in place. It preserves the existing class colors, sequential overlap blending, labels, and reading-order annotations. The unused full-page mask helper is removed and its OpenCV dependency guard moves to draw_mask.

The change is limited to paddlex/inference/models/layout_analysis/result.py. It uses the existing LayoutAnalysisResult._to_img contract, where the mask canvas has the same dimensions as the image.

Validation

CPU comparison against the unmodified develop implementation, using OpenCV 4.10.0, NumPy 2.3.5, and Pillow 12.3.0:

• 50 saved real document pages: complete rendered images are pixel-identical.
• 1,515 synthetic cases: blended pixels and complete images are identical. Cases cover clipping, concave and self-intersecting polygons, degenerate polygons, fractional coordinates, repeated classes, overlaps, small images, and smaller mask canvases.
• 15,000 additional rasterization comparisons, including coordinates from -4096 to 4095: identical blended pixels.
• A 21 × 21 polygon region on an 800 × 600 image allocates a 21 × 21 mask.
• Black, Flake8, isort, license headers, dependency-import checks, Python 3.8 annotation checks, and git diff --check.

Earlier isolated service measurements

The same patch was measured on PaddleX 3.7.2 with one A800 80 GB, all image outputs enabled, 16 warmup pages, and three repetitions over the same 50 pages. Values below are medians in seconds. Both sides already included the same four earlier pipeline/backend/client optimizations; only local mask rendering was added for this comparison.

Requests	Baseline	Local mask rendering
50 sequential single-page requests	82.498	57.125
One 50-page request	62.955	33.735

These are prior controlled measurements, not a fresh end-to-end benchmark of this branch or an unmodified PaddleX baseline. Rendering concurrency and other performance changes are outside this PR.