ToneMappingEffect is always converted to a separate full screen pass

[gkjohnson]

As noted in #304, I've noticed that the ToneMappingPass is incorrectly marked as a "convolution" effect, which is inconsistent with the attributes provided in the upstream "postprocessing" effect class and with the definition of the convolution effect attribute enum ("Convolution" here means the pass requires multiple samples from the effect which is not the case here). With this setting and the loop change from #250 simply adding a ToneMapping effect can easily turn what should be a single render pass into three render passes.

Again it looks like this change was made in #245 without a lot of exact reasoning. If there are cases where the tonemapping effect is incompatible with other effects then certainly there must be better ways to fix this rather than forcing multiple passes onto any user using tonemapping.