opt: batch trail rendering and fix ribbon twisting

[MikePehel]

Summary

• Batch trail draw calls: Replace per-segment DrawLine3D and DrawTriangle3D with single rlBegin/rlEnd batches using rlVertex3f/rlColor4ub. Reduces draw call overhead from ~3600 per vehicle (at max trail capacity) to 1 for directional trails and 1 for speed ribbons.
• Fix ribbon twisting on tight turns: Add perpendicular vector blending (70% previous frame + 30% current) to the speed ribbon so quad edges flow smoothly through sharp direction changes instead of overlapping/twisting.

Details

Both trail modes (directional lines and speed ribbon triangles) now submit all geometry in a single rlBegin/rlEnd pair per vehicle instead of issuing individual Raylib draw calls per segment. The color and vertex data are identical — only the submission path changes.

The ribbon blending works by lerping each segment's perpendicular vector toward the previous segment's perpendicular before computing quad corners. This produces a smooth, continuous ribbon surface even on tight orbits.

Only src/vehicle.c is modified. Uses rlgl.h which is already included. No new dependencies. Cross-platform (Mac/Windows/Linux).

Test plan

• Verify directional trail (T key, mode 1) renders identically to before
• Verify speed ribbon (T key, mode 2) renders without twisting on tight turns
• Confirm no visual regression in any view mode
• Test with multiple vehicles to verify per-vehicle batching