fromCubemap( cubemap, renderTarget = null ) {

		return this._fromTexture( cubemap, renderTarget );

	}
Generates a PMREM from an cubemap texture, which can be either LDR
or HDR. The ideal input cube size is 256 x 256,
as this matches best with the 256 x 256 cubemap output.

@param {Texture} cubemap - The cubemap texture to be converted.
@param {?WebGLRenderTarget} [renderTarget=null] - The render target to use.
@return {WebGLRenderTarget} The resulting PMREM.