Fix RoughnessMipmapper

[elalish]

Some of the recent changes in three.js have broken the RoughnessMipmapper: I was getting errors like this in Firefox where the GL errors are a bit more verbose than Chrome:

WebGL warning: generateMipmap: Tex image TEXTURE_2D level 0 is incurring lazy initialization.
WebGL warning: drawElementsInstanced: TEXTURE_2D at unit 1 is incomplete: Bad mipmap dimension or format.

I've got it all working now in MV using this change. It changes the behavior of FramebufferTexture a bit, so that it allocates during uploadTexture and then only fills in data during copyFramebufferToTexture, instead of doing the allocation there. This is important because I call this function in a loop to fill each mip level, and I think the reallocations were the problem.

Anyway, @Mugen87 @mrdoob please give this some scrutiny, since I'm not sure what else this might affect.

[mrdoob]

Thanks!