WebGLMorphtargets: Dispose morph textures when corresponding geometry is disposed.

[0b5vr]

Original discussion: https://discourse.threejs.org/t/how-do-i-free-morph-textures/33033

Description

Morph textures are not disposed when disposing corresponding geometries.

In WebGLMorphtargets, since each morph textures are one-to-one to each geometries and it only referred from this part,
it can be safely disposed once geometry is disposed.

I have added an event listener of 'dispose' to geometry that disposes the morph texture.
This will ensure morph textures be freed once it's no longer necessary.

[0b5vr]

I've built the three.js with this patch and made a demo on glitch: https://glitch.com/edit/#!/three-r133-vtf-morph-memory-leak

[Mugen87]

To clarify, morph textures are maintained in a weak map. So if there are no references to the geometry, the texture should automatically disappear. However, like mentioned at the forum this solution is more consistent compared to other parts of the code base.

[mrdoob]

Thanks!