Example: Clean up

[WestLangley]

Mostly minor clean up...

...but I also changed the default metalness to 0, because the HDR effect -- the point of the example -- is not perceptually different from the LDR case when the metalness is 1.

[WestLangley]

Unrelated: The tonemapping exposure is set like so:

renderer.toneMappingExposure = Math.pow( params.exposure, 4.0 );

This produces unreasonably overexposed results for the specified [ 0, 2 ] exposure range. I know this was copied from elsewhere, but is there a reason for the power relationship here? Don't we want perceptually-linear behavior? Why not simply:

renderer.toneMappingExposure = params.exposure;

/ping @bhouston for your thoughts...

[bhouston]

The equation is my fault.

There are two different exposures. I believe that internally to Three.JS we should keep exposure linear ( https://en.wikipedia.org/wiki/Exposure_(photography) ) but sometimes people like to have perceptual like controls and that is generally an exponeitial ( https://en.wikipedia.org/wiki/Exposure_value ).

Although the equation for the exponential exposure is more like ( Math.pow( 2, exposureValue ) * D + C ) = exposure, but I haven't yet researched exactly what D and C should be. That should be fairly simple to figure out.

[WestLangley]

Thanks @bhouston. We can deal with it in a separate PR.

This can be merged, then.

[mrdoob]

Thanks!