3D draw point generation

[Rewryte]

Summary

Bugfixes "3D draw point generation"

Purpose of change

Tiles draw sprites on screen by generating a draw_point for each grid. To handle visibility differences on different z-levels in 3D vision, draw_points recalculate their visibility variables on each z-level. However, there are situations where a grid that should not have a draw_point could inherit a draw_point from the grid above it which would cause the grid to show up as visible when it should not.
Fix #69973

draw_points generation now also includes caching and "occlusion culling" features, replacing my previous code. Doing these during draw_points generation is not only more efficient, but also substantially reduces code clutter.

Describe the solution

• draw_points are now generated separately for each z-level.
• draw_points no longer track the lowest level it should draw. If a grid should not be drawn, it would not have a draw_point in the first place.
• draw_points, overlay_strings and color_blocks are now cached, which will reduce cpu load per draw. Visibility variables are contained in draw_points and are no longer cached separately.

Describe alternatives you've considered

draw_points are stored in a map within a map (std::map<int, std::map<int, std::vector<tile_render_info>>>). Not sure if there is a more efficient way of storing the data.

Testing

Compiles ok.
Confirmed that #69973 is fixed.
Debug overlays work normally.
Up to 13% faster draw time when displaying multiple z-levels.

Additional context

[Qrox]

Thanks for fixing this!