workaround for Pixel PMREM

[elalish]

Related issue: google/model-viewer#3336

The error was also visible on this example: https://threejs.org/examples/?q=hdr#webgl_materials_envmaps_hdr If you used a Pixel phone and set metalness = 1 and roughness = ~0.1 (this was only to make it the most obvious, and it's still kind of subtle):

Note the faint black lines in the sky, following the corners of the cube map.

Anyway, I'm pretty sure I found at least two different Pixel GPU driver bugs, but I could not pin down what was causing them. I did find two workarounds though:

1. Increasing the padding on the cube sides from half a pixel to a full pixel fixed the visible seams.
2. While I was debugging and had PMREM blur disabled, I noticed a bizarre artifact in the lower mips where I was really not getting the right data (again, only on Pixel). It was probably masked by all the blurring on top of it, but I found that giving the texture a little padding on the top mostly fixed it, so I went ahead and included that here too, since who knows when it might become a visible artifact.

@mrdoob, thanks for noticing, though I wish I had a more satisfying answer.

[mrdoob]

Thanks!