⚡ Optimize matrix drawing with OpenCV

[google-labs-jules]

This PR optimizes the _draw_thick_line method in slither_env.py by replacing the manual circle interpolation loop with optimized OpenCV calls (cv2.line and cv2.circle).

Changes

• Imported cv2 in slither_env.py.
• Replaced the implementation of _draw_thick_line to use cv2.line for the main segment and cv2.circle for the rounded endpoints (capsule shape).
• Updated README.md to include opencv-python-headless in the installation command.

Performance

Benchmark results for 1000 iterations (2000 calls):

• Original: ~2.34s (~1.17ms per call)
• Optimized: ~0.02s (~0.01ms per call)
• Speedup: ~117x

Verification

• Visual comparison confirmed that the new implementation produces a nearly identical output (capsule shape) with only minor anti-aliasing differences.
• Existing tests (tests/test_env.py) passed.

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PR created automatically by Jules for task 14236199338200424516 started by @dzaczek

[google-labs-jules]

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