Optimized Mesh Merging

[lawnjelly]

Changes from PoolVector to LocalVector and pre-reserving vectors rather than push_back.

Tests show more than factor of 3 speedup for merging 200,000 boxes.

Notes

• LocalVector is generally more efficient than PoolVector for single threaded operations.
• Although mesh merging is already available through rooms & portals and the MeshInstance::merge_meshes() function, it may be exposed further in 3.6 so it makes sense to make it as fast as possible, especially for use at runtime at level load, rather than pre-baking.

Side issue - copy initialization in LocalVector

I noticed that both when assigning a Vector to LocalVector and also for assigning PoolVector, it seems to have to be done as two expression as follows, rather than LocalVector blah = PoolVector(foo);. I just copied the code for assigning Vector, but it would be nice if this can be solved:

	LocalVector<Vector3> normals;
	normals = PoolVector<Vector3>(arrays[VS::ARRAY_NORMAL]);

If you try it as a single expression there is a compile error.

EDIT: Solved the above, the problem was this needs copy initialization rather than assignment operator overload. LocalVector needed an operator LocalVector::LocalVector(const PoolVector<T> &p_from). I've also added one for LocalVector::LocalVector(const Vector<T> &p_from) as I had previously noted this expression didn't work. The PR is now changed to use this short form rather than the longer form assignment quoted above.

[clayjohn]

Great work! Its amazing to see the difference using LocalVector can make. We will have to keep our eyes open for other similar places that will benefit.

[akien-mga]

Thanks!