Fix WebGLState leak with EquirectangularToCubeGenerator, PMREMGenerator, PMREMCubeUVPacker

[jsantell]

Fixes #15288.

Due to the nature of the APIs of EquirectangularToCubeGenerator, PMREMGenerator, and PMREMCubeUVPacker, to generate multiple textures, a new instance is needed for each, where currently, each instance of these generators utilize their own scene/camera, accumulating WebGLStates for each render (see #15288 for details). This only addresses the root of the memory leak, not any of the API changes in #15288.

This change wraps each generator in a closure so that multiple instances of a generator reuse the same scene/camera, eliminating the WebGLState accumulation. The shader is also reused in EquirectangularToCubeGenerator and PMREMGenerator, and cloned in PMREMCubeUVPacker, which while the webgl program should be reused across instances, there was still some accumulation with regard to the instantiation of new materials.

Here's a rather pathological test case of generating a new EquirectangularToCubeGenerator, and passing through a PMREMGenerator and PMREMCubeUVPacker once every second; drop it in examples/:

https://gist.github.com/jsantell/c1e13139cfdd8526ba28a5078d608464
(L155-L194 is the loop of interest)

Before this patch, most memory that is allocated is retained across 3 minutes and things started to slow down:

After this patch, nothing beyond compiled code is generated and retained, or at least, far less over the course of 8 minutes:

Post-patch, the total retained memory hovered around 6MB, taking GC into consideration. Previously, it was hard to tell due to more GC spikes and seemingly larger peaks, but best case several more MBs.

While this could be an extreme scenario, and wasn't my main concern, the amount of allocation size saved after this patch is 98%.

Can confirm with webgl_materials_envmaps_[exr|hdr].html examples for correctness

[mrdoob]

Thanks!