Add heading to enginePreloadWorldArea, extract (some of) LoadScene logic

[Timic3]

Summary

This pull request adds an optional heading argument to the existing enginePreloadWorldArea, and stops it from calling the game's CStreaming::LoadScene/LoadSceneCollision. Previously, it called the game's CStreaming::LoadScene directly, which besides "preloading the area" also:

1. Unloaded every streamed in object in the game
2. Dropped all pending model requests
3. Loaded zone vehicle models, cop models and path nodes for the area
4. Loaded radar tiles for the area - I did not include it in this PR due to unloading the tiles for current position, causing the radar to flicker

enginePreloadWorldArea(float x, float y, float z [, string loadingOption = "all", float heading = nil])

NOTE: This will only load models in the specified heading, not collisions!

Motivation

For Sphene project (single player in MTA) we needed to load a world area in specific direction, like LOAD_SCENE_IN_DIRECTION does in SCM.

Test plan

Some examples, extracted from the SCM:

2026-10-06.20-25-43.mp4

Checklist

• Your code should follow the coding guidelines.
• Smaller pull requests are easier to review. If your pull request is beefy, your pull request should be reviewable commit-by-commit.