This was caused by the SSR option resetting the accumulation. But the
render passes were only cleared in the init phase. This means that
when SSR was resetting the taa_render_sample the actual renderpasses
would still contains 1 sample. This means the renderpasses were always
divided by the wrong number of samples.

The fix is to clear just before accumulation if the sample is 1.

The fact that it works for motion blur is kind of a blessing. This is because
we check stl->effects->ssr_was_valid_double_buffer before resetting the
sampling. So this only happens on the first motion step and does not affect
the rest of the rendering.