screen-space reflections are integrated in the indirect light computation
(similarly to screen-space refraction), this is because it can be seen as
the specular term of screen-space GI.
However, because this effect is screen-space, the data it accesses has
the ibl luminance (and exposure) applied.
Besides some minor refactoring, this change just applies the
screen-space reflections *after* the ibl's specular term is exposed.
3de427b74f