BufferAttribute: Introduce dispose().

[Mugen87]

This PR replaces #15308. Fixed #15261.

A first step to make BufferAttribute fixed sized.

The new policy should be: If you need to resize buffer data, you have to create a new instance of BufferAttribute.

[WestLangley]

/bump overlooked?

[Mugen87]

Resolved the merge conflicts.

[0b5vr]

I think I need this right now!

[Mugen87]

@mrdoob Any chances to merge the PR? If not, do you mind sharing your concerns about this change? Maybe we can find a different solution if this one is not satisfying?

[OndrejSpanel]

Note: there is currently no method to properly dispose instanceMatrix and instanceColor in the InstancedMesh.

I think this is really a glaring hole in the resource management. A proper cleanup should always be possible. Someone may have a scene with a lot of dynamic instancing going on (I am now experimenting with exactly such scene) and the missing cleanup causes slowing growing allocation of WebGL resources in such case.

[Mugen87]

After a deeper analysis I do not vote to introduce BufferAttribute.dispose() anymore since this easily breaks code in WebGLBindingStates. Geometry data are managed as a single entity. Calling dispose() on attribute level and thus deleting the WebGL buffer would not automatically update the respective VAO.

InstancedMesh.dispose() has to be implemented without BufferAttribute.dispose().