Rendering: Pre-allocate light vectors in scene culling

[kellemar]

While looking through the rendering code, I noticed that directional_lights and lights_with_shadow vectors in _render_scene are built up with push_back() without reserving capacity first. Since we already know the upper bound from scenario->directional_lights.size(), we can avoid the reallocations.

It's a small thing, but this runs every frame during scene rendering so it adds up.

Changes

• Added reserve() for directional_lights before the loop
• Added reserve() for lights_with_shadow inside the directional lights block

Both use scenario->directional_lights.size() as the capacity hint, which is the maximum number of elements either vector can have.

[clayjohn]

I would also move the arrays to be member arrays of the class so they can be reused across frames and we can avoid making new allocations each frame.

In general we discourage making performance PRs without any profiling numbers (https://contributing.godotengine.org/en/latest/engine/guidelines/optimization.html). However, in this case it is acceptable since this also aligns with our goal of having 0 allocations per frame.

That's why I suggest taking this a step further and actually eliminating the per-frame allocations.