Replace HWBufferMap with a list and back pointers

[paradust7]

Fixes minetest/minetest#12231 (comment)

[sfan5]

Tested, looks like it works fine.

[lhofhansl]

I find it hard to reason about whether HWBuffers are indeed released as soon as a mesh is not longer visible.
That was the crux of the original problem, HWBuffers would hang around for all cached meshes.

When the mesh's reference count is 0 that means the mesh is not visible?

[sfan5]

It doesn't depend on the visibility of meshes but uses the lifetime of the mesh buffer.
When a hardware buffer is added it keeps an extra reference to the mesh, then when the reference count of the mesh drops to 1 both the hardware buffer and the last reference to the mesh are dropped.

[lhofhansl]

Thanks @sfan5 . So that means that each cached mesh will keep its HWBuffer around, instead of just the visible meshes?

In any case, I'll try it out when I get some time next.

[sfan5]

So that means that each cached mesh will keep its HWBuffer around, instead of just the visible meshes?

Yes. This doesn't sound like a problem to me since the unload timeout on the client will get rid of old data at some point.